Build an additional kwin_gles binary

KWin gets compiled twice if OpenGL ES is available
and a second binary is created. It is linked against
the GLES effects and kwinglesutils and does not link
against OpenGL. The normal OpenGL variant is still
the "kwin" binary.

This is an intermediate step till we have the scene
as a loadable plugin. Then we will determine whether
OpenGL ES is supported in an external application
and load it as a plugin, otherwise we load the
OpenGL/GLX scene as a plugin.

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