missing: better tips about missing tools

missing: better tips about missing tools

* lib/missing: Try to point the user to the home page of such tools
where possible.  Also, make clear that some tools (e.g., aclocal or
autoheader) are distributed as part of bigger projects (resp. automake
and autoconf, in those cases).  While at it, refactor and re-format
the existing code heavily, in particular introducing
(give_advice): ... this new function in the process.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>