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
fix Issue 11966 - inout(const(char))[] doesn't convert to inout(char)[]
fix Issue 11974 - ICE(cast.c) Segfault with invalid assignment
Merge pull request #3141 from 9rnsr/fix11974

Issue 11974 - ICE(cast.c) Segfault with invalid assignment
Merge pull request #3128 from MartinNowak/fix11956

fix Issue 11956 - dmd doesn't lookup /etc/dmd.conf
Merge pull request #3138 from 9rnsr/fix11966

[REG2.065a] Issue 11966 - inout(const(char))[] doesn't convert to inout(char)[]