Add support for building libplasma with GLES2

Review Request #100705 - Created Feb. 21, 2011 and submitted - Latest diff uploaded

Jammy Zhou
kdelibs, plasma
After build kwin with GLES2 code path, the kwin binary still has dependency on, which is introduced by Then we also need to add GLES2 support to libplasma, so that kwin/plasma only has dependency on in this case. 

The new option "BUILD_PLASMA_WITH_OPENGLES" added in attached patch is disabled by default, and distributions can turn it on when do packaging for OpenGL ES2.0 support.
After turn the "BUILD_PLASMA_WITH_OPENGLES" option on, kwin-gles has no dependency on now. And by default, libplasma still depends on when run ldd.