relax SVN archive

This problem was reported by Douglas Kojetin <douglas dot kojetin at gmail dot com> at
https://mail.gna.org/public/relax-users/2006-11/msg00002.html (Message-id:
<AD17DD2D-A37B-44D4-B1B8-1A2349EC7EC5@...>) and as bug #7670 (https://gna.org/bugs/?7670).

To use a different suffix, I have explicitly set the 'SHLIBSUFFIX' environmental variable to '.so'.
This should overwrite the SCons default of '.dylib'.