WIP: Vendor the SQLite amalgamation instead of the autoconf tarball

[flavorjones]

WIP - this PR exists to get feedback on builds across all supported platforms.

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Context

Since v3.48.0, SQLite's "autoconf" release does not actually use autoconf, and as a result there have been small related breakages including:

• gem install sqlite3 --platform=ruby fails to build starting at 2.6.0 #634
• Database#load_extensions is not enabled on Windows when using native gems or compiling vendored source code (sqlite 3.48.0+) #618

I suspect that moving to the amalgamation and explicitly setting compiler and linker flags from within the extconf.rb might be more reliable.

[flavorjones]

Blurgh, need to do some work around feature detection in order to keep the capability to link against a system library; using have_func is of course not going to work if we're compiling the amalgamation into the extension.