[CMake] Creation of CMAKE_*_LIBRARY_EXTENSION

[Rodolfo Lima]

I've been playing around with bug #5997 and I've found out the problem,
which lies in the fact that when the user sees the variable
CMAKE_(STATIC|SHARED|IMPORT)_LIBRARY_SUFFIX, he assumes that this is
something to be added to the library name (doesn't involve its
extension). But cmake assumes that this is the library's extension.

Wouldn't it be better to create a CMAKE_*_LIBRARY_EXTENSION to have
either '.lib' or '.a', and let CMAKE_*_LIBRARY_SUFFIX just be what its
name means? For cmake, the library name (including extension) would be:

${CMAKE_*_LIBRARY_PREFIX}${libname}${CMAKE_*_LIBRARY_SUFFIX}${CMAKE_*_LIBRARY_EXTENSION)

Regards,
rod