Using GL for the PKG_CHECK_MODULES identifier multiple times means only
the first call will actually be used. Later calls will be skipped due to
GL_CFLAGS and GL_LIBS already being set. This changes DRI to using a
different identifier and DMX to just reusing GL_CFLAGS.
Signed-off-by: Dan Nicholson <dbn.lists@gmail.com>
e1c8ee2157