Remove unnecessary LLVM_DIR variable. find_package() will create this
for us.
Remove useless comments.
Remove hack to get the directory containing the LLVM tools by using
LLVM_TOOLS_BINARY_DIR variable which is available in the released
LLVM3.5
Use the LLVM_ENABLE_RTTI CMake variable available in the released
LLVM3.5 to determine if we also need to disable RTTI.
Silence CMake 3.0 warnings about reading LOCATION property of targets.
This should be fixed properly at some point.
Add name argument to add_cxx_flag() macro so that what is being
tested is more descriptive in CMake's configure output.
Warn if clang is not available