Support for using SVN checkout of LLVM.
Support for building LLVM/LDC on MinGW.

Uses Ninja mainly because the MinGW shell would always mis-escape
the -G "MSYS Makefiles" argument...
Specify LLVM_ROOT_DIR instead of LLVM_CONFIG for Windows compatibility.

llvm-config does not exist in a CMake build there.
Hardcode libconfig location on MinGW.

Fetching and building libconfig should be included as a
separate step.
Correctly replace config file paths on MinGW.
MinGW package creation adaptions.