meson: Expose library build type as global variables

Given that it can be computed using an error-prone strings comparisons it
is better to provide a variable everywhere, so that we don't have the
risk of comparing values that are always false.

gobject/meson.build

@@ -62,7 +62,7 @@ gobject_sources += files(
   'gvaluetypes.c',
 )
 
-if host_system == 'windows' and get_option('default_library') == 'shared'
+if host_system == 'windows' and glib_build_shared
   gobject_win_rc = configure_file(
     input: 'gobject.rc.in',
     output: 'gobject.rc',