let qt tools can link clang and llvm statically or respective libs

Some llvm distributions may provide the static libraries, link qt tools statically may reduce the number of dependencies, and some distributions libclang and libclang-cpp may not include all components.
 
Now qt tools may use 

• clangHandleCXX
• clangBasic
• clangLex
• clangFrontend
• clangTooling
• clangSerialization
• clangAST
• LLVMSupport

update:

 If LLVM is configured with -DLLVM_BUILD_LLVM_DYLIB=OFF, the libLLVM will be not available, must use libLLVMSupport etc.

It would be more flexible if there is a CMake variable like 
 CLANG_LINK_CLANG_DYLIB in clang, let user choose link against libclang-cpp or clang's components.

Example is on Windows, there is no libLLVM.dll or libclang-cpp.dll (except the LLVMSharp), just only other static libraries.