Add --installed-libs option, allowing packagers to build parts of svn (usually bindings) against installed svn libraries.
* Makefile.in (swig-pl_DEPS): Remove libsvn_client libsvn_delta libsvn_diff libsvn_fs libsvn_ra libsvn_repos libsvn_subr libsvn_wc so that swig-pl can be built against installed libraries, as py and rb already can.
* build/generator/gen_make.py (Generator.write): Load gen-make.opts and save the list of installed libraries in self.installed_libs. For libraries in the list, write out -l lines instead of references to the .la file, and don't attempt to build that library.
* gen-make.py (_usage_exit): Document --installed-libs option. (): And parse it from sys.argv.