Add in missing $PTHREAD_LIBS to all backends that can make use of it.
Rename --enable-fork-process to --enable-pthread to maatch internal
logic. Also, stop linking in pthreads needlessly if its been
disabled.
doc/Makefile to use automake. Main difference is that
no longer installs man pages for backends that are not
compiled. Moved install ownership of $top_srcdir docs
to $top_srcdir's Makefile.