fix Issue 11956 - dmd doesn't lookup /etc/dmd.conf

- define variable in src/posix.mak instead of posix.mak
- fix quotes
- assert that SYSCONFDIR was set to a non-empty string

[Issue 11956 – dmd doesn't lookup /etc/dmd.conf](https://d.puremagic.com/issues/show_bug.cgi?id=11956)