Make C++ compiler configurable

The C++ compiler used to build prepare-builtins
may differ from the llvm/clang for which we are
building libclc.

Use 'clang++' as the default compiler.

Patch by: Jeroen Ketema

git-svn-id: https://llvm.org/svn/llvm-project/libclc/trunk@193220 91177308-0d34-0410-b5e6-96231b3b80d8
Clean-up dependency files

Patch by: Jeroen Ketema

git-svn-id: https://llvm.org/svn/llvm-project/libclc/trunk@193221 91177308-0d34-0410-b5e6-96231b3b80d8