Hi Mike,
(also answering your "further building question" :p) rebuilt the application, with support for OS X 10.5 (10.6 and higher were already supported): http://www.ionic.de/x2goclient-3.99.1.0-release-r2.dmg
Thanks to Stefan for being able to test this stuff on his 10.5 Mac!
Seems like there is a weird memory leak on 10.5, but other than that it seems to work fine.
Best regards,
Mihai