[glib-networking] Fix gio_module_dir with some wrong gio-2.0.pc

[Michael Catanzaro <mcatanzaro src gnome org>]

commit d6f11c97baef0cb610fdad0c80a8721943103f7d
Author: Xavier Claessens <xavier claessens collabora com>
Date:   Mon Dec 10 21:01:24 2018 -0500

    Fix gio_module_dir with some wrong gio-2.0.pc
    
    For example fedora 29's gio-2.0.pc defines libdir=/usr/lib64 instead of
    libdir=${prefix}/lib64

 meson.build | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/meson.build b/meson.build
index 1a3bef2..aa6f83d 100644
--- a/meson.build
+++ b/meson.build
@@ -57,10 +57,10 @@ if glib_dep.type_name() == 'internal'
   gio_module_dir = glib_proj.get_variable('glib_giomodulesdir')
 else
   gio_module_dir = gio_dep.get_pkgconfig_variable('giomoduledir',
-                                                  define_variable: ['prefix', prefix])
+                                                  define_variable: ['libdir', join_paths(prefix, libdir)])
 endif
 
-assert(gio_module_dir != '', 'GIO_MODULE_DIR is missing from gio-2.0.pc')
+assert(gio_module_dir.startswith(prefix), 'GIO_MODULE_DIR is missing from gio-2.0.pc')
 
 # *** Checks for LibProxy   ***
 libproxy_dep = dependency('libproxy-1.0', version: '>= 0.3.1', required: get_option('libproxy'))