meson: Use proxy-libintl if gettext is not found

This is a stub-only library that can be used while building against
MSVC and contains no i18n machinery at all.

The dependencies added indirectly use the libintl.h header, and when
built as a subproject, the header won't be in a path known the
pre-processor.

gio/win32/meson.build

@@ -10,5 +10,6 @@ giowin32_sources = [
 giowin32_lib = static_library('giowin32',
   sources : [giowin32_sources, gioenumtypes_h],
   include_directories : [configinc, glibinc, gioinc, gmoduleinc],
+  dependencies : [libintl],
   pic : true,
   c_args : [ '-DHAVE_CONFIG_H', '-DG_DISABLE_DEPRECATED' ] + gio_c_args)