Add option to use ninja to build cmake projs

* configure.ac (--enable-ninja): Add new option.
* src/gpg4win.mk.in: Update KDEPKG and TOOLSPKG templates
to use ninja if requested.

The difference on my system for a clean build is:

With ninja:
11530,39s user 1172,79s system 1362% cpu 15:32,65 total
With GNU/Make
11529,29s user 1241,82s system 1257% cpu 16:55,62 total

So not that much but noticeable.