docs: make yelp usage conditional in manual

The help/manual directory will be recursed into (so make dist can
include everything) even if not in SUBDIRS in help/Makefile.am. Because
of this help/manual/Makefile.am is always used to generate the
help/manual/Makefile. And so if help/manual/Makefile.am includes
unconditional yelp usage then help/manual/Makefile will also include
yelp usage, even if yelp is not available.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=719644