Explicitly pass -std=gnu++03 to the compiler.

This fixes the build with GCC 6, which defaults to -std=gnu++14. The port's
code does not work with C++11 or later.

Annoyingly, this port's build system mixes CFLAGS and CXXFLAGS, so we need to
pass -std=gnu++03 via CFLAGS for things to work properly.

PR: 219275