Generate manpages in build directory.

Generate manpages in build directory.

It is not necessary to generate the manpages in the source directory
during a split build; ‘make dist’ can still find them in the build
directory and put them in the tarball.

Also add some defensive logic to the .x.1 rule to ensure that
bin/command and tests/command exist before generating man/command.1.
Without this, if you do a sufficiently parallel build, help2man may
generate the manpage from an older installed copy of ‘command’.
(Ideally, we wouldn’t have to run ‘command’ at all and this would not
be an issue, but ‘help2man’ doesn’t appear to support that.)

After this patch, the only files written to the source directory
during the ‘make’ phase of a split build (starting from a clean Git
checkout) are

doc/version.texi
doc/stamp-vti
doc/autoconf.info
doc/standards.info

These are not under our control, they’re being created by automake’s
built-in rules for Texinfo documentation.

* man/local.mk: Replace all instances of $(mansrcdir) with literal ‘man’.
  (.x.1): Ensure that bin/command, tests/command, and the man
  directory exist before creating man/command.1.