Prepend the srcdir to the Version_script path so building from an

custom directory works again. This didn't matter previously as 
Version_script was created by configure, which put it in the build 
directory and thus a local copy was available.

Also apply it only on Linux and Solaris, instead of whenever GNU ld
is used as before. This is probably not exactly correct, since the
options are the same on Solaris, and GNU ld probably always accepts
--version-script, but as we're close to release, I chose a more
conservative resolution of the ambiguity in the previous code.

svn path=/trunk/theora/; revision=13879