The previous behaviour was to add a CXX flags when
msdev/devenv/nmake/VCExpress was detected.
However these tools have nothing to do with the compiler used.
The /W4 applies only when CL (aka MS-VC) is the current compiler.
It avoids a compilation issue when nmake is used with another
toolchain than the MS one (eg. the Android NDK which is base on GCC,
and of course GCC do not know the /W4 flag!).
Use MSVC instead of MSVC_VERSION variable to do the test.

Change-Id: I287796dcba9c39a8f912db10bd8b8f527a544bdb