Do not assign to VALAFLAGS

Do not assign to VALAFLAGS

Users should be able to do "make VALAFLAGS=--enable-experimental-non-null"
without overriding necessary flags like --target-glib=2.24. The variable
for use by package maintainers is AM_VALAFLAGS.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=695628
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Philip Withnall <philip@tecnocode.co.uk>