Include ERROR_CFLAGS everywhere we compile C code

Include ERROR_CFLAGS everywhere we compile C code

AM_CFLAGS was missing from a couple of foo_CFLAGS variables (it isn't
"inherited" automatically), so add that too.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=697354
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Philip Withnall <philip@tecnocode.co.uk>