Xcode: Build fixes/improvements

* Fixed 64-bit iOS build (changed ARCHS back to $(ARCHS_STANDARD).)
* Don't run the build script (configure, and generate sqlite3.c) if the
  file 'sqlite3.c' already exists and is newer than 'configure'.
* Note that you still have to build for OS X first, because that's the
  only platform for which the configure script will work. Once that's
  built and sqlite3.c generated, you can build the other destinations.
Xcode: Enable FTS and RTree features