Create OpenGL::GL and OpenGL::GLU imported targets using the locations
found.
This feature was originally added by commit v3.1.0-rc1~420^2~2
(FindOpenGL: Provide imported targets for GL and GLU, 2014-05-31) but
had to be reverted by commit v3.1.0-rc3~10^2 (FindOpenGL: Revert support
for imported targets, 2014-12-01) due to issue #15267. Since then we
added support for `IMPORTED_LIBNAME` to interface libraries, so use it
to handle the case where we have only the library name without an
absolute path.
Inspired-by: Philipp Möller <bootsarehax@googlemail.com>
Closes: #15267
