WIP: clang-cl support for win32-static/win64-static buildenvs

[mkrautz]

This is an experimental PR for adding clang-cl support to the Windows build.

Mostly, it's a matter of:

• For Makefile-based builds, to abstract away the calls to cl.exe and perhaps link.exe to a variable ($MUMBLE_CC, maybe), to be able to target the LLVM binaries
• For CMake-based builds, we probably need to change recipes to use the vcproj generator, and use the -T parameter to select the platform toolset (LLVM-vs2014). The chnage to the vcproj generator might break our ability to "install" our builds, like we currently do with nmake install, but I am not sure.
• For MSBuild-based builds, we should just need to pass the correct toolset, which happens already.
• For everything else, a combination of the above

Other things to work out:

• Our Windows buildenvs use static libraries all the way through. From a cursory glance at the OpenSSL build, no $TOOLSET.pdb files (i.e., v120.pdb -- object PDB files) were generated during the build. Perhaps clang-cl doesn't support that at the moment, or maybe it embeds it into the .objs. Who knows. It would be best if it just emitted $TOOLSET.pdbs, so we didn't have to special case anything...

TODOs:

• nasm -- OK, binary
• cmake -- OK, binary
• jom -- OK, binary
• python -- OK, binary
• perl -- OK, binary
• wix -- OK, binary
• boost -- Need to figure out how to build with clang-cl using b2.exe
• openssl -- OK, still uses MS link.exe
• protobuf
• libogg
• libvorbis
• libflac
• libsndfile
• bonjour
• zlib
• libpng
• libjpeg-turbo
• sqlite3
• mariadb-client
• freetype.build
• harfbuzz-ng.build
• pcre.build
• qt5.build
• libmcpp.build
• bzip2.build
• berkeleydb.build
• expat.build
• zeroc-ice.build
• asio -- OK, header only
• g15sdk -- OK, binary

[mkrautz]

Not actively working on this. I just wanted to throw it out here.