fix the only place where CFLAGS are used with CXX
fix the only place where CFLAGS are used with CXX

everywhere else, except this line, it's correctly using
CXXFLAGS.

fredrik-johansson/flint2,fredrik-johansson/flint2,wbhart/flint2,wbhart/flint2,fredrik-johansson/flint2,wbhart/flint2