Merge branch 'jc/freebsd-without-c99-only-build'

FreeBSD 13.0 headers have unconditional dependency on C11 language features, and adding -std=gnu99 to DEVELOPER_CFLAGS would just break the developer build. * jc/freebsd-without-c99-only-build: Makefile: FreeBSD cannot do C99-or-below build