Makefile.in: only build and install man pages for supported binaries

This makes it possible again to install from the git repo even if not
all binaries can be build, e.g. Mac OS which always lacks para_fade
(reverts a858de7cf9437cf9a3569eb4bc297c5b0e768994).

Also, don't refuse to install and don't complain if $VARDIR
(/var/paraslash) can not be created, as this is non-fatal.