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
configure: print when checking for yelp-tools

https://bugzilla.gnome.org/show_bug.cgi?id=719645
make: create subdirectories of the content_files to the builddir

Fixup for "make: copy the files with their relative path included" also
by me.
That is it is fine to keep the subdir from the source to the build
directory, only it could not already have been generated. Do so.
Required by a followup patch for PackageKit which has SETUP_FILES
in a the "spec" directory. As nothing is generated there, it needs
action to create it in the build tree.

https://bugzilla.gnome.org/show_bug.cgi?id=720061