cmake/OpenCVGenPkgconfig.cmake: rework opencv.pc generation

Using absolute path to locate the components in the "Libs:" field of the
*.pc can badly break cross-compilation, especially when building
statically linked objects.

So, this patch replace the components' absolute path by the form:
  -L<libdir> -l<libname>

This way, the linker will be able to resolve each dependency path,
whatever the kind of objects/build (shared object or static build) it
is dealing with.

Note that for static link, the library order does matter. The order of
the opencv components has been carefully chosen to comply with this
requirement.