fix check if gcc is actually clang

New macro FP_CHECK_GCC_IS_CLANG checks to see if the compiler given
with the argument --with-gcc is actually clang.

What probably still needs to be done is to enhance the check to take --with-cpp into account and automatically set the correct flags. That would, however, only work for gcc's CPP and clang'c CPP and not for other CPPs. Perhaps we should make --with-cpp-flags mandatory when --with-cpp is given.

/Peter