build: Clean up uses of INCLUDES and _CPPFLAGS

INCLUDES is deprecated in favour of AM_CPPFLAGS and per-target
_CPPFLAGS. Since AM_CPPFLAGS is ignored in preference to a per-target
_CPPFLAGS variable, we should define the latter only when it is needed.

Fixes: https://bugzilla.gnome.org/726570
build: Use Makefile.introspection

Fixes: https://bugzilla.gnome.org/726570