0.9.2 --- a standard header compiles for every target, not only Linux

[Sunrisepeak]

A consumer translation unit that includes a standard header did not compile for
the macOS or Windows target:

__locale_dir/support/linux.h:86:12: error: no member named 'strtof_l' in the global namespace
__locale_dir/support/linux.h:237:17: error: no member named 'vasprintf' in the global namespace

This configuration's locale support calls strtof_l, strtod_l, strtold_l
and vasprintf; musl declares them only under _GNU_SOURCE; and Clang
predefines _GNU_SOURCE for C++ on Linux targets and on no other. import std
never showed it, because the std module is compiled with this package's own
flags. Every module wrapper of a header library does show it, because the
wrapper includes the library's headers in its own translation unit. Found
building nlohmann.json from mcpp-index over this runtime for
x86_64-windows-gnu and aarch64-macos.

• llvm-generated/generic/__config_site states _GNU_SOURCE. It is included by
  <__config> before any C header, including libc++'s wrappers of the C headers.
• CI cross-builds examples/cxx (which includes <filesystem>) for both
  targets beside same-source. Measured locally: without the change both builds
  fail with the errors above; with it both succeed, and the Linux run of the
  example still reports failures: 0.
• Version 0.9.2; README records the new line among the configuration's claims.