meson: Build header tests with -pedantic

meson: Build header tests with -pedantic

By using __extension__, we can silence pedantic errors we cannot or
do not want to fix.

This in particular silences:
 - enum values being outside of int range
 - variadic macros
 - long long being C99
 - type of bit-field ‘type’ is a GCC extension
 - use of C99 bool in public header functions