all Makefiles: don't ignore or clean .c files not corresponding to Vala

all Makefiles: don't ignore or clean .c files not corresponding to Vala

This will become more important if we use C to implement fast-paths
and faster data structures.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=695381
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Philip Withnall <philip@tecnocode.co.uk>