gnutls: don't crash if G_TLS_GNUTLS_PRIORITY is invalid

If G_TLS_GNUTLS_PRIORITY can't be parsed, print a warning and fall
back to the default value.

https://bugzilla.gnome.org/show_bug.cgi?id=701693
2.37.4
gnutls: require gnutls 3.0
gnutls: Fix deprecation warnings

https://bugzilla.gnome.org/show_bug.cgi?id=697908
build: use AM_CPPFLAGS instead of INCLUDES
Update glib.mk, build installable tests
Add missing m4 file to fix build breakage
Revert "gnutls: require gnutls 3.0"

There are potential licensing issues with gnutls 3.x, which
transitively requires gmp, whose latest versions are LGPLv3.

Allow compiling with 2.x again, but bump up the requirement to 2.12,
so we've got all the newer APIs to replace the ones that are
deprecated in 3.x.

This may be revisited in the future, and some new code added to this
module will probably be gnutls-3.x-only.

https://bugzilla.gnome.org/show_bug.cgi?id=704503
2.37.5