build: Stop defining STDC_HEADERS
build: Stop defining STDC_HEADERS

This was previously defined by the AC_HEADER_STDC macro in configure.ac,
but we don’t use that any more. Nothing in GLib depends on this macro,
and neither does anything in my /usr/include which we might care about.

The autoconf documentation for AC_HEADER_STDC says it’s deprecated:
https://www.gnu.org/software/autoconf/manual/autoconf-2.67/html_node/Particular-Headers.html#index-AC_005fHEADER_005fSTDC-621

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://gitlab.gnome.org/GNOME/glib/issues/1313

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib