pkg-config variant to use internal glib (#2104)

Pkg-config depends on glib which depends on pkg-config. As a result,
pkg-config used to build glib internally. However, this fails on Mac.
Building pkg-config with an internal glib is now a variant, turned on
by default, and required to build glib.

var/spack/repos/builtin/packages/glib/package.py

@@ -42,7 +42,7 @@ class Glib(Package):
     depends_on('autoconf', type='build')
     depends_on('automake', type='build')
     depends_on('libtool', type='build')
-    depends_on('pkg-config', type='build')
+    depends_on('pkg-config+internal_glib', type='build')
     depends_on('libffi')
     depends_on('zlib')
     depends_on('gettext')