m4/buildsys.m4: remove -mmacosx-min-version (added by MacPorts Portfile as needed.)
Partitially revert 433b0f5e94804e25dbd7bf21ff16b3135f3e8cfd, minus removal of the silent flag.
buildsys.mk.in: merge in changes from upstream https://github.com/Midar/buildsys.