Use USR_CXXFLAGS for c++ compiler flags

Avoid warnings like:

warning: command-line option ‘-std=c++11’ is valid for C++/ObjC++ but not for C

requested review from @gabrielfedel

I see there was an issue (#1 (closed)) about this. Alfo was right, USR_CPPFLAGS is for C preprocessor flags.

You are right! It is fine for me to merge this!

mentioned in commit af58a7eb

merged