Makefile: Fix for {Net,Free,Open}BSD and generic $CFLAGS in environ

• CFLAGS ?= inherits from the environment, so shouldn't have -std=c99
• In practice everything but Darwin uses -shared
• NetBSD and MacOS have a broken CMSG_DATA macro when -D_POSIX_C_SOURCE=200809L

is set their bug but probably better to make it work for now, specially in
MacOS case