WL#1100: Move charset related defines into a separate charset.h.in

Mark wrote:

I'm still also waiting for a fix that I've discussed informally with
Monty (and I'd appreciate your input), where I asked if it would be
possible to separate out non-OS-specific configuration options (like
charsets) into a separate .h file, using autoconf (I'm not an autoconf
guru, so I'm not sure how feasible it is). The problem is that with each
 'bk commit' in the 4.1 tree, the Windows team is finding yet one more
thing that is thrown into my_config.h, which Windows builds don't (and
can't) use because it contains os-specific #defines as well as other
stuff, thus leading to a maintenance nightmare