Adding operating system detection to configure for bug 1209

[Umberto Nicoletti]

I am working on:

http://mapserver.gis.umn.edu/bugs/show_bug.cgi?id=1209

and I need to figure out the os name to use the proper include path
for the java headers.
I have added a new configuration option to configure called
--with-os-name which will set a variable in mapscript/java/Makefile.in
allowing the correct directory to be added to the include path. The
patch has been tested by user and reported to work.

Since this could be useful for others too I accept suggestions on:

- configure option name and explanation text
- os checking code: should we use the AC_CANONICAL_SYSTEM macro instead?

Thanks for your attention,
Umberto