component/glib: make build depend on python version

We should not use $PYTHON, because this implicitly depends on the
python version that was used when the shared part is installed, leading
to this kind of problematic scenarios:
 - install the part from a python2.7 software, this creates scripts
  with "python2.7" in shebang line
 - install a python3 software referencing glib, it uses the shared
  part that was installed with python2, but "python2.7" is not in $PATH

By referencing the part explicitly, it becomes part of the signature
of the part, so a software on python2 and a software on python3 will
use a different version of the shared part.

component/glib/buildout.cfg

@@ -7,6 +7,7 @@ extends =
   ../perl/buildout.cfg
   ../xz-utils/buildout.cfg
   ../zlib/buildout.cfg
+  ../defaults.cfg
 parts =
   glib
 
@@ -25,7 +26,7 @@ configure-options =
   --disable-fam
   --disable-xattr
   --disable-man
-  --with-python=$PYTHON
+  --with-python=python${python:version}
 environment =
   PATH=${gettext:location}/bin:${patch:location}/bin:${perl:location}/bin:${xz-utils:location}/bin:%(PATH)s
   CPPFLAGS=-I${gettext:location}/include -I${zlib:location}/include