build: BSD make support for yat2m.

* configure.ac (YAT2M): Only define when found.
* doc/Makefile.am: Portability fix.

This is not intended to apply to master, but 2.2 branch only. When
new libgpg-error is required, installation of yat2m can be assumed.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>