Log Message:
Fix Linux build by respecting CFLAGS during configure. Remove unneeded
workaround for FreeBSD 6. NFCI on other platforms. Build-tested on:

- NetBSD 9.2 and -current (9.99.92)
- Tribblix m25.1
- FreeBSD 13
- CentOS 7 and 8
- Devuan 4
- Debian 9, 10, 11
- Ubuntu 16.04.7 LTS, 18.04.6 LTS, and 20.10