fixup! glib: put the expected python in $PATH

The OS may not have Python (slapos-node package does not depend
on any version of Python). Another possible issue was found on Suse:

  make[3]: Entering directory '.../parts/glib__compile__/glib-2.56.4/gio'
    GEN      gdbus-daemon-generated.c
  Traceback (most recent call last):
    File "./gdbus-2.0/codegen/gdbus-codegen.in", line 53, in <module>
      from codegen import codegen_main
    File ".../parts/glib__compile__/glib-2.56.4/gio/gdbus-2.0/codegen/codegen_main.py", line 30, in <module>
      from . import parser
    File ".../parts/glib__compile__/glib-2.56.4/gio/gdbus-2.0/codegen/parser.py", line 23, in <module>
      import xml.parsers.expat
  ImportError: No module named xml.parsers.expat

component/glib/buildout.cfg

@@ -15,6 +15,11 @@ recipe = slapos.recipe.cmmi
 shared = true
 url = https://ftp.gnome.org/pub/gnome/sources/glib/2.56/glib-2.56.4.tar.xz
 md5sum = 17c3dca43d99a4882384f1a7b530b80b
+# --with-python=${buildout:executable} is simpler but we may end up with
+# scripts whose shebang exceeds the kernel limit.
+# And ${buildout:executable}/.. is not a valid $PATH part.
+configure-command =
+  PATH=`dirname ${buildout:executable}`:$PATH exec ./configure --prefix=@@LOCATION@@
 configure-options =
   --with-python=python
   --disable-libmount