Use -Wno-sign-conversion if g++ is secretly clang

Full disclosure: I have very minimal Makefile experience, so this may need to be tweaked

Attempts to resolve issue where compilation using clang (which is what g++ seems to actually use on recent versions of macOS) is more sensitive to -Wsign-conversion warnings (e.g. "error: implicit conversion changes signedness: 'int' to 'size_type'")