* Suggest users to use VS on Windows
* Add a top-level wrapper Makefile and disallow in-source builds
* Support clang
* Centralize compiler flags
* Remove all occurrences of "if(WIN32)..."
* Make qrenderdoc an external project
* Building qrenderdoc on windows will be supported just because that's
likely where I will be developing it mostly, but for the foreseeable
future it won't be built by default. If it ever gets good enough to
replace the .NET UI, then we can look at it again.