cogl/configure.ac: AC_SUBST an empty MAINTAINER_CFLAGS
cogl/configure.ac: AC_SUBST an empty MAINTAINER_CFLAGS

For compatibility with the way we build Cogl as part of Clutter we now
substitute an empty MAINTAINER_CFLAGS variable. When building Cogl
standalone all our extra CFLAGS go through COGL_EXTRA_CFLAGS so the
separate MAINTAINER_CFLAGS aren't used, but automake will get confused
if a substitution isn't made.

gcampax/cogl,collects/cogl,djdeath/cogl,spatulasnout/cogl,djdeath/cogl,Distrotech/cogl,spatulasnout/cogl,gcampax/cogl,Distrotech/cogl,gcampax/cogl,djdeath/cogl-android,djdeath/cogl-android,Distrotech/cogl,gcampax/cogl,Distrotech/cogl,djdeath/cogl-android,djdeath/cogl,collects/cogl