CMake: Introduce LLVM_CLANG_SOURCE_DIR, "tools/clang" by default. Clang will not be built if LLVM_CLANG_SOURCE_DIR="" or ${LLVM_CLANG_SOURCE_DIR}/CMakeLists.txt is not found.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142103 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed