Fix build problem on Solaris

-rpath only takes one dash
Fixes the Solaris build \o/

'''
<fraggeln> SaberUK: http://m.theo.nu/jenkins/job/InspIRCd-2.0/435/label=solaris/console <--
<fraggeln> do you have any clue on why?
<@Shutter> Hmm. Where did that extra - come from.
<fraggeln> Shutter: no idea :)
[...]
<fraggeln> Shutter: sub make_rpath($;$) <-- in make/utilities.pm
<fraggeln> thats where that extra - is'''
Merge pull request #784 from ShutterQuick/patch-1

Fix build problem on Solaris