Allow Ruby to be compiled statically but use dynamic extensions

This might be a solution to a problem that doesn't need solving, but I was trying to install MRI 1.9.3 on CentOS 5.10 with the `--static` flag, and the installation kept failing during the `make` stage when any of the extensions that were uncommented from `ext/Setup` were compiled.  It looks like this was due to `make` trying to run `make` in each of the extension directories, which was failing due to there not actually being a `Makefile` present.  While there's probably a simple solution for that, the simpler solution in my case, since I was trying to recreate a hand-compiled Ruby that was compiled with the `--disable-shared` flag but did not use statically compiled extensions, was to add a `--dynamic-extensions` flag that causes RVM to skip over the updating of the `ext/Setup` file that would otherwise happen when the `--static` flag is passed.  This allows RVM to pass the `--disable-shared` flag without forcing static compilation of extensions when called with the `--static --dynamic-extensions` flags.