mod-xslt uses some ISO C99 constructs. Is the error because of C99?
If yes, you should really update your compiler. It has been 14 years since the standard was approved,
and most version of gcc since 2004 have supported enough of it to compile modxslt.
If no, maybe you are missing some dependencies? Check out the wiki or the mailing list,
or file a bug. Visit www.modxslt.org for more details.