Instead of attempting to figure out what the paths will be, take the
returned file object and pass that around, meson will then automatically
figure out the correct paths.
The latter doesn't work correctly when being used as a subproject, as it
returns the *absolute* root. So if pkgconf is being built as part of
muon, then it will return muon's source root. current_source_dir, on the
other hand returns the directory correctly whether being built as a
subproject or superproject.
autoconf uses either C99 or Gnu99. Meson does not provide a graceful way
to select gnu99 if possible or c99 (though there are several proposals
currently happening to get there), so I've selected c99 as the
conservative default. Without this, the compiler uses whatever it's
default happens to be, which may or may not work out correctly, and
hides bugs from CI that are present with c99 as the default.
Dylan Baker