Re-brand ERROR_CFLAGS to C_ERROR_CFLAGS

Re-brand ERROR_CFLAGS to C_ERROR_CFLAGS

These are flags that we only use when compiling hand-written C code,
mostly originating from telepathy-glib. We can't be that strict with
the C code generated by Vala, because it's full of questionable things
(which the Vala compiler verifies are OK, but the C compiler still warns
about).

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>