For simplicity it's better to avoid conditionals that rely on stuff in
bsd.port.pre.mk or bsd.port.options.mk. Also, as no one complained
about non-working *FLAGS, so I've removed them instead of:
CXXFLAGS_amd64= -fPIC CXXFLAGS_sparc64= -O0
amd64 tried to pass cxxflags via BJAM_ARGS more than once while
sparc64 cannot be fixed without altering sanitizing in
BJAM_ARGS+= cxxflags="${CXXFLAGS:N-O*}" cflags="${CFLAGS:N-O*}"