The best way to compile is to use out of src build, like such:

mkdir some/path/build
cd some/path/build
cmake -DGALLIUM_DIR=/home/user/projects/mesa /home/user/projects/clover
make

That should be it, make sure that GALLIUM_DIR point to top of Mesa checkout. If your Clang is installed to a different location than LLVM, then define CLANG_INCLUDE_DIR and CLANG_LIB_DIR like so: -DCLANG_INCLUDE_DIR=/opt/clang/include -DCLANG_LIB_DIR=/opt/clang/lib