FindOpenGL: need name-only imported targets on Windows

This issue was created automatically from an original Mantis Issue. Further discussion may take place here.

On window the OpenGL libraries are linked by library name rather than filename. For visual studio OPENGL_gl_LIBRARY is for example set to opengl32.

The IMPORTED_LOCATION of the imported target OpenGL::GL target is also set to opengl32. Since IMPORTED_LOCATION is expected to be a path this fails with e.g. ninja at build time with:

ninja: error: 'opengl32', needed by 'bin\foobar.dll', missing and no known rule to make it

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