0.9.2

Fixes a defect every consumer of this package that targets macOS or Windows is
exposed to: a translation unit that includes a standard header did not compile
for aarch64-macos or x86_64-windows-gnu.

__locale_dir/support/linux.h:86:12: error: no member named 'strtof_l' 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 only. 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 — nlohmann.json from
mcpp-index is where it was found.

• __config_site states _GNU_SOURCE, so every target has what the Linux target had.
• CI cross-builds examples/cxx, which includes <filesystem>, for both targets.