Stop exporting mandir that used to be exported only when
config.mak.autogen was used.  It would have broken installation of
manpages (but not other documentation formats).

* jc/remove-export-from-config-mak-in:
  Fix `make install` when configured with autoconf
  Makefile: do not export mandir/htmldir/infodir
  config.mak.in: remove unused definitions