glib: use same Python as buildout

Sometimes, glib is only used as a dependency of pkg-config,
which means it's not used by the instantiated services.

So do not force building Python if slapos.rebootstrap is not used.

component/glib/buildout.cfg

@@ -14,7 +14,7 @@ recipe = slapos.recipe.cmmi
 url = http://ftp.gnome.org/pub/gnome/core/3.22/3.22.2/sources/glib-2.50.2.tar.xz
 md5sum = 5eeb2bfaf78a07be59585e8b6e80b1d6
 configure-options =
-  --with-python=${python2.7:location}/bin/python2.7
+  --with-python=${buildout:executable}
   --disable-libmount
   --disable-static
   --disable-selinux