Bug 1032528 - Use CFLAGS when determining the compiler version in case it can be affected by what is there; r=glandium

This for example allows us to use -fmsc-version in CFLAGS/CXXFLAGS
with clang-cl.
Bug 1034094 - Remove support for the gcc-style clang driver on Windows; r=glandium

This basically reverts the now unneeded parts of bug 752034.
clang-cl driver should be used on Windows instead.