Don't build cogl-gles2-gears if configured with --disable-cogl-gles2
Don't build cogl-gles2-gears if configured with --disable-cogl-gles2

The cogl-gles2-gears example will fail to build unless
--enable-cogl-gles2 is enabled in the configure script because it
depends on libcogl-gles2. This patch makes it only conditionally build
the example depending on the BUILD_COGL_GLES2 automake conditional.

Reviewed-by: Robert Bragg <12e9293ec6b30c7fa8a0926af42807e929c1684f@linux.intel.com>

djdeath/cogl-android,djdeath/cogl-android,gcampax/cogl,djdeath/cogl-android,djdeath/cogl,gcampax/cogl,djdeath/cogl,gcampax/cogl,gcampax/cogl,djdeath/cogl