Add CMake support for using clang++ as the C++ compiler

There was a weird issue where CHECK_CXX_SOURCE_COMPILES wouldn't find
clock_gettime under clang++, switching to CHECK_FUNCTION_EXISTS worked
both under clang++ and g++ on Ubuntu 13.10
Merge pull request #1520 from alexmalyshev/master

Add CMake support for using clang++ as the C++ compiler