Project using win32-clang-msvc mkspec should build using nmake or jom

When using a win32-clang-msvc mkspec, qmake generates Makefiles for nmake. When building a project with an LLVM kit on Windows, Qt Creator attempts to use an instance of GNU make detected in my PATH. This results in errors from GNU make when it tries to parse Makefiles intended for nmake. Qt Creator should instead use whichever nmake or jom it would usually use for an MSVC kit.