build: fix race in parallel builds

build: fix race in parallel builds

As reported by Hongxu Jia:
> The automake-$(APIVERSION) is a hardlink of automake, if it is
> created later than update_mans executing, there is a failure
> [snip]
> |: && mkdir -p doc && ./pre-inst-env /usr/bin/env perl ../automake-1.16.1/doc/help2man --output=doc/aclocal-1.16.1 aclocal-1.16
> |help2man: can't get `--help' info from aclocal-1.16
> |Try `--no-discard-stderr' if option outputs to stderr
> Makefile:3693: recipe for target 'doc/aclocal-1.16.1' failed
> [snip]
>
> The automake_script is required by update_mans and update_mans
> invokes automake-$(APIVERSION) rather than automake to generate
> doc, so we should assign `automake-$(APIVERSION)' to automake_script.
>
> The same reason to tweak aclocal_script.

However, rather than update the _script variables to point to the
hardlinked copies of the programs, we can have the help2man steps
run the existing scripts directly.  This makes the relationship a
bit more explicit and avoids implicit dependencies on names.

* doc/local.mk: Pass $(aclocal_script) and $(automake_script) to $(update_mans).
* THANKS: Add Hongxu Jia.