Use gnustep-config if available

Use gnustep-config if available

* configure.ac (--with-gnustep): Document this.
(NS_GNUSTEP_CONFIG): New variable, set if gnustep-config works.
If gnustep-config works, use 'gnustep-config --objc-flags' and
'gnustep-config --gui-libs' to compute GNUstep configuration
variables, instead of attempting to infer them individually.
* etc/NEWS: Document this.
* src/emacs.c [NS_IMPL_GNUSTEP]: Don't include <GNUstepBase/GSConfig.h>.
It doesn't appear to be needed, and the inclusion breaks on
Ubuntu 14.10 when 'configure' uses 'gnustep-config'.
Fixes: bug#19507