Filter out GCC-ism for Clang

At least mariadb-5.5.34-2.fc20.x86_64 "mysql_config --cflags" somewhat
unhelpfully includes GCC-specific -fstack-protector-strong which at least Clang
trunk towards 3.5 does not understand.

Change-Id: Ic38104323b6d9275384effdf21d1885c0720edb0
Work around G_DEFINE_TYPE -Werror,-Wunused-function

At least G_DEFINE_TYPE from glib2-devel-2.38.2-2.fc20.x86_64
/usr/include/glib-2.0/gobject/gtype.h defines unsed *_get_instance_private
functions.

Change-Id: I47211b6451d9699c7b8741555f3ad11a09e411bf