Bug#948134: gnome-shell-mailnag FTCBFS: stores valac in CC

[Jeremy Bícha <jeremy.bicha@canonical.com>]

This change seems to cause the package to fail the build. Before your
change, this is a build warning not an error.

dh_auto_build -- CC=valac
make -j8 "INSTALL=install --strip-program=true" CC=valac
make[2]: Entering directory '/<<PKGBUILDDIR>>'
valac --pkg folks aggregate-avatars.vala
aggregate-avatars.vala:53.3-53.41: warning: unhandled error `GLib.Error'
   53 | cache_dir.make_directory_with_parents();
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Unknown option -I/usr/include/glib-2.0
Run 'valac --help' to see a full list of available command line options.
error: cc exited with status 256
Compilation failed: 1 error(s), 1 warning(s)


By the way, I am pushing a 40.0-4 upload to Unstable now. But this is
a QA package so you're welcome to directly upload a fix for this
issue.

Thank you,
Jeremy Bícha