fix: Suppress obsolete -single_module linker warning on macOS

[cpsievert]

Summary

Fixes a WARNING from R CMD check on newer macOS toolchains (Apple clang 21+, SDK 26.4+), as seen on Brian Ripley's M1 Mac CRAN check machine:

* checking whether package 'httpuv' can be installed ... [75s/89s] WARNING
Found the following significant warnings:
  ld: warning: -single_module is obsolete

Root cause

libuv's bundled configure script (via libtool) probes for -single_module linker flag support by compiling a test program with -Wl,-single_module. On newer macOS, the linker accepts the flag but emits ld: warning: -single_module is obsolete. The configure script actually handles this correctly — it greps stderr for "single_module" and sets lt_cv_apple_cc_single_mod=no. But the grep command itself prints the matched line to stdout, which leaks into 00install.out and gets flagged by R CMD check as a significant warning.

Fix

Pre-set lt_cv_apple_cc_single_mod=no on the libuv configure command line. This causes configure to use the cached value and skip the probe entirely, so the problematic linker invocation never happens.

This is safe because:

• It's the same value configure would arrive at on affected systems (the probe detects the warning and sets it to no anyway)
• On older macOS, -single_module has been the default linker behavior for years, so omitting the flag is a no-op
• Pre-setting autoconf cache variables is a standard, well-documented technique for working around configure probes

Test plan

• Clean R CMD INSTALL with USE_BUNDLED_LIBUV=true succeeds
• R CMD check passes with Status: OK (no WARNING)
• Verified single_module does not appear in 00install.out

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR suppresses a macOS-specific ld: warning: -single_module is obsolete warning emitted during the bundled libuv ./configure probe, preventing it from leaking into 00install.out and triggering an R CMD check WARNING for httpuv builds on newer Apple toolchains.

Changes:

• Pre-seed libuv’s autoconf cache variable lt_cv_apple_cc_single_mod=no when invoking ./configure to skip the -single_module probe.

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[shikokuchuo]

My understanding is that on non-mac platforms, pre-setting this variable would be a no-op, hence all good!