Make gnu symlink creation work again

Since the default configure_args is now expanded immediately it cannot depend
on variables defined later.
To fix this for users of gnu_link_progs another API change is necessary.

gnu_link_progs is now a private array of programs to link. The
private gnu_link function is renamed do_gnu_link. A new public function
named gnu_link is added which handles populating gnu_link_progs and
modifying configure_args.

buildpkg.packaging.solaris

@@ -96,7 +96,7 @@ _mandir=share/man
 _infodir=share/info
 
 # Default configure args
-configure_args=(${gnu_link_progs:+--program-prefix=$gnu_prefix} --prefix=$prefix --mandir=${prefix}/${_mandir} --infodir=${prefix}/${_infodir})
+configure_args=(--prefix=$prefix --mandir=${prefix}/${_mandir} --infodir=${prefix}/${_infodir})
 
 # Host specific configuration
 [ -r $buildpkgscripts/config.`hostname`.solaris ] && . $buildpkgscripts/config.`hostname`.solaris