Qmake is executed every time when using Clang toolchain

• Create a Clang-on-MinGW kit on Windows
• Create a minimal qmake project
• Build it
• Build again - watch how qmake is executed every time although nothing has changed

The root cause is that the Makefile parser doesn't parse the sysroot and targetTriple-related arguments, and QmakeBuildConfiguration::compareToImportFrom compares then against userArgs, which don't match.