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')

var/spack/repos/builtin/packages/pkg-config/package.py

@@ -36,17 +36,21 @@ class PkgConfig(Package):
     version('0.28',   'aa3c86e67551adc3ac865160e34a2a0d')
 
     parallel = False
+    variant('internal_glib', default=True,
+            description='Builds with internal glib')
 
     # The following patch is needed for gcc-6.1
     patch('g_date_strftime.patch')
 
     def install(self, spec, prefix):
-        configure("--prefix={0}".format(prefix),
-                  "--enable-shared",
+        args = ["--prefix={0}".format(prefix),
+                "--enable-shared"]
+        if "+internal_glib" in spec:
             # There's a bootstrapping problem here;
             # glib uses pkg-config as well, so break
             # the cycle by using the internal glib.
-                  "--with-internal-glib")
+            args.append("--with-internal-glib")
+        configure(*args)
 
         make()
         make("install")