ngircd: Add support for GLOBAL_PASSWORD

We need to disable PAM support at compile time to get this working,
because with PAM compiled in global password are broken until 485d0ae
(Use server password when PAM is compiled in but disabled, 2014-02-27,
[1]).  485d0ae landed after ngIRCd v21, so it will probably be in the
next release after that.  Once that lands in Gentoo's Portage tree, we
can re-enable PAM.

[1]: http://arthur.barton.de/cgi-bin/gitweb.cgi?p=ngircd.git;a=commit;h=485d0aec813db9966922f17aae044df2d82b0b67