Add a CXXWARNFLAGS variable

Some warning flags are valid for C++ but not C. GCC 8 complains if you pass
such flags when building a C file. Using a separate variable for these
flags allows building both C and C++ files in the same directory (such as
the fusefs tests) under GCC.

Reviewed by: cem, emaste
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21116