VS6: Add handling of CMAKE_*_LINKER_FLAGS_<CONFIG> variables

Read the content of the config specific linker flags in the
Visual Studio 6 generator too.
Xcode: Use upper case version of the config when reading the library flags

Add a missing call to cmSystemTools::UpperCase() to get the upper case
version of the configuration name needed by GetStaticLibraryFlags().
Merge topic 'static_library_flags' into next

bcfee5c Xcode: Use upper case version of the config when reading the library flags
9a591ec VS6: Add handling of CMAKE_*_LINKER_FLAGS_<CONFIG> variables