Add "gles3" as a new api in PIGLIT_TOP/cmake/target_api. Now, you can
build GLES3 tests and libraries by placing the build rules in files named
CMakeLists.gles3.txt.

I dislike the CMake hackery of havining multiple builds for each target
api. But, because GLES2 and GLES3 use different headers (gl2.h vs
gl3.h), until piglit-dispatch gains support for GLES the most sensible way
to add GLES3 support to piglit is to continue the tradition of adding
a new target api into PIGLIT_TOP/cmake/target_api.

Signed-off-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>