giscanner: undef "GLIB_VERSION_{MAX_ALLOWED,MIN_REQUIRED}" macros in generated C file
When g-ir-scanner is used by another project, than that project might have the GLIB_VERSION_* macros defined. This is useful to ensure that only intended glib API is used.
The project might then also pass the CFLAGS to g-ir-scanner, without filtering those defines out. This can lead to compiler warnings.
For example, NetworkManager sets the version macros to GLIB_VERSION_2_40 and thus gets these warnings
/NetworkManager/tmp-introspect66917zc4/NM-1.0.c: In function ‘dump_object_type’:
/NetworkManager/tmp-introspect66917zc4/NM-1.0.c:252:13: warning: Not available before 2.70
252 | if (G_TYPE_IS_FINAL (type))
| ^~~~~~~~~~~~~~~~~
/NetworkManager/tmp-introspect66917zc4/NM-1.0.c: In function ‘dump_fundamental_type’:
/NetworkManager/tmp-introspect66917zc4/NM-1.0.c:370:13: warning: Not available before 2.70
370 | if (G_TYPE_IS_FINAL (type))
| ^~~~~~~~~~~~~~~~~
But these warnings are not correct. The installed g-ir-scanner knows for which glib version to generate code. Undefine the macros to avoid the warning.