Yuni::Process::Program: fixed invalid header guard
cmake: fixed compilation woth clang on Linux

Clang does not seem to like the option -stdlib on Ubuntu, but seems
required on MacOS (Investigation ongoing)
Enable YUNI_{UN}LIKELY (via gcc builtin __builtin_expect) when available

Latest versions of clang support some gcc builtins, like
__builtin_expect (to provide the compiler with branch prediction
information). The presence of this builtin is now detected by the configure.