Fix mingw build script.

LLVM 3.5 requires use of C++11. The gcc compiler requires a flag to use C++11.
The easiest way to pass this flags is to uses the flags returend by llvm-config.
Using llvm-config with mingw requires some path fiddeling because of the mixed
Windows and Unix conventions.
Merge pull request #774 from redstar/mingw

Fix mingw build script.