Add a new macro for configuring projects, LLVM_CONFIG_PROJECT. This macro

takes care of the --with-llvmsrc and --with-llvmobj options for the project
It was moved here from the project's configure.ac file because there is
some tricky handling of the llvm_src variable to tell the project where the
llvm source tree is (for mkinstalldirs and install-sh commands).

llvm-svn: 20303