Tame -Wall on MSVC and improve Clang/C2 support

Tame -Wall on MSVC and improve Clang/C2 support

MSVC's -Wall has more in common with clang's -Weverthing than gcc's -Wall. The result is very overwhelming (3000+ lines of warnings).
From now on, if building with MSVC *and* if a a .devel is found *and* if MSVC is not using clang as it's front end, increase the warning level to 4. Else do the regular  check_and_add_compiler_option() routine.

It is probably also worth noticing that cmake's compiler identification string for LLVM/Clang is *not* true when using the Clang/C2 toolset. Only something like `if(MSVC AND NOT ${CMAKE_C_COMPILER} MATCHES "clang*")` or `if(CMAKE_GENERATOR_TOOLSET STREQUAL "v140_clang_3_7")` can detect it.