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.

gio/tests/gdbus-object-manager-example/meson.build

@@ -39,7 +39,7 @@ gdbus_example_objectmanager_rst_gen = custom_target('objectmanager-rst-gen',
 )
 
 extra_c_args = []
-if get_option('default_library') == 'static'
+if glib_build_static_only
   extra_c_args = '-DGDBUS_OBJECT_MANAGER_EXAMPLE_STATIC_COMPILATION'
 endif