Unify the way the flags of a static library are read

Introduce cmLocalGenerator::GetStaticLibraryFlags() to have a central
function for getting the linker flags for a given target.
Add support for CMAKE_STATIC_LINKER_FLAGS

Add the content of this variable to the target specific linker flags
as we do with the other CMAKE_*_LINKER_FLAGS variables already.
Add CMAKE_STATIC_LINKER_FLAGS to CMakeCommonLanguageInclude

Add support for CMAKE_STATIC_LINKER_FLAGS_* to
CMakeCommonLanguageInclude.cmake to set the defaults similar
to the other CMAKE_*_LINKER_FLAGS_*.
Add documentation for the missing CMAKE_*_LINKER_FLAGS_* variables

Add CMAKE_MODULE_LINKER_FLAGS_*, CMAKE_SHARED_LINKER_FLAGS_* and
CMAKE_STATIC_LINKER_FLAGS_* to cmDocumentVariables.cxx with a
similar documentation as CMAKE_EXE_LINKER_FLAGS_*.
Add additonal tests for the linker flags

Extend Tests/LinkFlags to test libraries of the type MODULE and
the content of the CMAKE_*_LINKER_FLAGS_* variables.
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.
Merge topic 'static_library_flags' into next

33e6e0b VS6: Add handling of CMAKE_*_LINKER_FLAGS_<CONFIG> variables
152dfda Add additonal tests for the linker flags
20ed496 Add documentation for the missing CMAKE_*_LINKER_FLAGS_* variables
54f7019 Add CMAKE_STATIC_LINKER_FLAGS to CMakeCommonLanguageInclude
2a43c30 Add support for CMAKE_STATIC_LINKER_FLAGS
14bbf83 Unify the way the flags of a static library are read