libxslt: don't pass --version-script to Solaris linker.

Fixes "ld: fatal: option --version-script requires option -z
gnu-version-script-compat to be specified" on Solaris 11.4.

Tested to build as before on Tribblix, and therefore hopefully SmartOS.
NFCI on non-"SunOS" platforms.

(schmonz)