fix up config.h generation: Standard headers are always included...

- We don't need to check if standard C headers (string.h, limits.h) etc.
  exist
- Check for some stuff we didn't include before (i.e. inttypes.h)

Change-Id: Ie92ffe75e319133518fee357cd69a762a665dc3a
Reviewed-on: http://review.couchbase.org/43789
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>