[CMake] No <LANG>_FLAGS_<CONFIG> target property.

[Óscar Fuentes]

There are target properties such as LINK_FLAGS and LINK_FLAGS_<CONFIG>,
but I don't see properties for setting compiler flags. This makes
impossible to build two targets on the same CMakeLists.txt with
different compiler flags (I was told that the last value of
CMAKE_<LANG>_FLAGS_<CONFIG> applies to all targets on that
CMakeLists.txt).

So why no per-target compile flags?