Some fix and clean-up stuff

Before improving snes9x (in term of features and GUI), my goal is to fix every warning so that we have a clean code.

When I started making changes, I found make targets to be incomplete some I added 'distclean' and updated 'clean' one.
I updated the configure script with a recent version of autoconf and started toying with configure options.
I found shared-memory was used even if dependencies were not met so I fixed that.
When I tried cross-compiling snes9x (to MIPS which is big-endian) to test my endiannes-related changes, I tweaked the configure script to disable some tests (because you can't run the binaries if cross-compiling) and to detect correctly X11.
I also added a 'Cross-compiling' section in readme_unix.html.
Finally, I fixed some warnings like '-Wdelete-non-virtual-dtor' and '-Wstrict-aliasing'.
I know the last changes introduce more code and seems unneeded but cleaner code is always better than some compact (and obfuscated) code.
Anyway, I tested my code (x86_64 and MIPS under qemu) and it work as before.

If these changes don't interest you, let me know.
Thanks