Check if libstdatomic is needed to link on the target platform

[sobomax]

Hi, thanks for the useful library. I am the author and maintainer of the Qemu fork allowing to run FreeBSD binaries to run unmodified on any modern Linux installation and recently somebody has submitted kqueue(2) emulation support using your library (https://github.com/sobomax/qemu-bsd-user-l4b).

Upon adding the library to the build I noticed 3 issues:

1. The GH Actions script is outdated, resulting in the test refusing to run (ubuntu-20.04 has been discontinued few months ago and 18.04 two years ago). See Update GitHub actions to use supported versions of the OS/toolchains #158 for the fix.
2. We would like to build a static library only. So I've added Add ENABLE_SHARED & ENABLE_STATIC config options #159 to add.
3. The linking would fail on linux/arm/v5 (debian:12), this is due to the platform not providing stdatomic as part of toolchain and needed an external library. To make matters worse, the static version of the said library is well hidden for some reason, requiring custom -L flag to dig it out. I've encountered this issue building arm/v5 version of other packages, so check for the need of -lstdatomic is added as well as some logic to provide path to the static library when desired.

#70 1610.9 FAILED: qemu-x86_64 
#70 1610.9 clang-19  -o qemu-x86_64 libcommon.a.p/gdbstub_syscalls.c.o libcommon.a.p/hw_core_cpu-common.c.o libcommon.a.p/hw_core_machine-smp.c.o libcommon.a.p/cpu-common.c.o libcommon.a.p/page-vary-common.c.o libcommon.a.p/disas_disas-host.c.o libcommon.a.p/disas_disas-target.c.o libcommon.a.p/disas_objdump.c.o libcommon.a.p/disas_disas-common.c.o libcommon.a.p/semihosting_stubs-all.c.o libcommon.a.p/accel_tcg_cpu-exec-common.c.o libcommon.a.p/accel_accel-user.c.o libcommon.a.p/common-user_safe-syscall.S.o libcommon.a.p/common-user_safe-syscall-error.c.o libqemu-x86_64-bsd-user.a.p/target_i386_tcg_user_excp_helper.c.o libqemu-x86_64-bsd-user.a.p/target_i386_tcg_user_seg_helper.c.o libqemu-x86_64-bsd-user.a.p/bsd-user_x86_64_signal.c.o libqemu-x86_64-bsd-user.a.p/bsd-user_x86_64_target_arch_cpu.c.o libqemu-x86_64-bsd-user.a.p/target_i386_cpu.c.o libqemu-x86_64-bsd-user.a.p/target_i386_gdbstub.c.o libqemu-x86_64-bsd-user.a.p/target_i386_helper.c.o libqemu-x86_64-bsd-user.a.p/target_i386_xsave_helper.c.o libqemu-x86_64-bsd-user.a.p/target_i386_cpu-dump.c.o libqemu-x86_64-bsd-user.a.p/target_i386_tcg_access.c.o libqemu-x86_64-bsd-user.a.p/target_i386_tcg_bpt_helper.c.o libqemu-x86_64-bsd-user.a.p/target_i386_tcg_cc_helper.c.o libqemu-x86_64-bsd-user.a.p/target_i386_tcg_excp_helper.c.o libqemu-x86_64-bsd-user.a.p/target_i386_tcg_fpu_helper.c.o libqemu-x86_64-bsd-user.a.p/target_i386_tcg_int_helper.c.o libqemu-x86_64-bsd-user.a.p/target_i386_tcg_mem_helper.c.o libqemu-x86_64-bsd-user.a.p/target_i386_tcg_misc_helper.c.o libqemu-x86_64-bsd-user.a.p/target_i386_tcg_mpx_helper.c.o libqemu-x86_64-bsd-user.a.p/target_i386_tcg_seg_helper.c.o libqemu-x86_64-bsd-user.a.p/target_i386_tcg_tcg-cpu.c.o libqemu-x86_64-bsd-user.a.p/target_i386_tcg_translate.c.o libqemu-x86_64-bsd-user.a.p/trace_control-target.c.o libqemu-x86_64-bsd-user.a.p/gdbstub_user-target.c.o libqemu-x86_64-bsd-user.a.p/cpu-target.c.o libqemu-x86_64-bsd-user.a.p/page-target.c.o libqemu-x86_64-bsd-user.a.p/page-vary-target.c.o libqemu-x86_64-bsd-user.a.p/fpu_softfloat.c.o libqemu-x86_64-bsd-user.a.p/accel_accel-target.c.o libqemu-x86_64-bsd-user.a.p/accel_tcg_tcg-all.c.o libqemu-x86_64-bsd-user.a.p/accel_tcg_cpu-exec.c.o libqemu-x86_64-bsd-user.a.p/accel_tcg_tb-maint.c.o libqemu-x86_64-bsd-user.a.p/accel_tcg_tcg-runtime-gvec.c.o libqemu-x86_64-bsd-user.a.p/accel_tcg_tcg-runtime.c.o libqemu-x86_64-bsd-user.a.p/accel_tcg_translate-all.c.o libqemu-x86_64-bsd-user.a.p/accel_tcg_translator.c.o libqemu-x86_64-bsd-user.a.p/accel_tcg_user-exec.c.o libqemu-x86_64-bsd-user.a.p/accel_tcg_user-exec-stub.c.o libqemu-x86_64-bsd-user.a.p/bsd-user_bsd-ioctl.c.o libqemu-x86_64-bsd-user.a.p/bsd-user_bsd-mem.c.o libqemu-x86_64-bsd-user.a.p/bsd-user_bsd-proc.c.o libqemu-x86_64-bsd-user.a.p/bsd-user_bsd-socket.c.o libqemu-x86_64-bsd-user.a.p/bsd-user_bsdload.c.o libqemu-x86_64-bsd-user.a.p/bsd-user_elfload.c.o libqemu-x86_64-bsd-user.a.p/bsd-user_main.c.o libqemu-x86_64-bsd-user.a.p/bsd-user_mmap.c.o libqemu-x86_64-bsd-user.a.p/bsd-user_thunk.c.o libqemu-x86_64-bsd-user.a.p/bsd-user_signal.c.o libqemu-x86_64-bsd-user.a.p/bsd-user_uaccess.c.o libqemu-x86_64-bsd-user.a.p/bsd-user_linux_os-proc.c.o libqemu-x86_64-bsd-user.a.p/bsd-user_linux_os-socket.c.o libqemu-x86_64-bsd-user.a.p/bsd-user_linux_os-stat.c.o libqemu-x86_64-bsd-user.a.p/bsd-user_linux_os-sys.c.o libqemu-x86_64-bsd-user.a.p/bsd-user_linux_os-syscall.c.o libqemu-x86_64-bsd-user.a.p/bsd-user_linux_os-thread.c.o libqemu-x86_64-bsd-user.a.p/bsd-user_linux_os-time.c.o libqemu-x86_64-bsd-user.a.p/bsd-user_linux_os-file.c.o libqemu-x86_64-bsd-user.a.p/bsd-user_linux_truss.c.o libqemu-x86_64-bsd-user.a.p/bsd-user_linux_os-dev_proc.c.o libqemu-x86_64-bsd-user.a.p/meson-generated_.._x86_64-bsd-user-gdbstub-xml.c.o libhwcore.a.p/hw_core_bus.c.o libhwcore.a.p/hw_core_qdev-properties.c.o libhwcore.a.p/hw_core_qdev.c.o libhwcore.a.p/hw_core_resetcontainer.c.o libhwcore.a.p/hw_core_resettable.c.o libhwcore.a.p/hw_core_vmstate-if.c.o libhwcore.a.p/hw_core_irq.c.o libhwcore.a.p/hw_core_clock.c.o libhwcore.a.p/hw_core_qdev-clock.c.o libqom.a.p/qom_container.c.o libqom.a.p/qom_object.c.o libqom.a.p/qom_object_interfaces.c.o libqom.a.p/qom_qom-qobject.c.o libevent-loop-base.a.p/event-loop-base.c.o gdbstub/libgdb_user.a.p/gdbstub.c.o gdbstub/libgdb_user.a.p/user.c.o tcg/libtcg_user.a.p/optimize.c.o tcg/libtcg_user.a.p/region.c.o tcg/libtcg_user.a.p/tcg.c.o tcg/libtcg_user.a.p/tcg-common.c.o tcg/libtcg_user.a.p/tcg-op.c.o tcg/libtcg_user.a.p/tcg-op-ldst.c.o tcg/libtcg_user.a.p/tcg-op-gvec.c.o tcg/libtcg_user.a.p/tcg-op-vec.c.o tcg/libtcg_user.a.p/perf.c.o -Wl,--as-needed -Wl,--no-undefined -static -fstack-protector-strong -Wl,-z,relro -Wl,-z,now -O1 -pipe -ffunction-sections -fdata-sections -DHAVE_BSD_STRING_H=1 -Wno-atomic-alignment -Wl,--gc-sections -Wl,--start-group -lbsd -lmd libqemuutil.a /usr/local/lib/libkqueue.a -lpthread -lrt /usr/lib/arm-linux-gnueabi/libelf.a -lm -pthread /usr/lib/arm-linux-gnueabi/libglib-2.0.a /usr/lib/arm-linux-gnueabi/libpcre2-8.a -Wl,--end-group
#70 1610.9 /usr/bin/arm-linux-gnueabi-ld: /usr/lib/arm-linux-gnueabi/libglib-2.0.a(gutils.c.o): in function `g_get_user_database_entry':
#70 1610.9 (.text+0x2b0): warning: Using 'getpwuid' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking
#70 1610.9 /usr/bin/arm-linux-gnueabi-ld: (.text+0xf0): warning: Using 'getpwnam_r' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking
#70 1610.9 /usr/bin/arm-linux-gnueabi-ld: (.text+0x130): warning: Using 'getpwuid_r' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking
#70 1610.9 /usr/bin/arm-linux-gnueabi-ld: /usr/local/lib/libkqueue.a(knote.c.o): in function `knote_new':
#70 1610.9 /tmp/libkqueue/src/common/knote.c:52: undefined reference to `__atomic_store'

With those 3 changes I have been able to build my Qemu fork against libkqueue and confirm it works on linux/386, linux/amd64, linux/arm/v5 and linux/arm64/v8. That being said, I have not tested any kqueue functionality specifically.