Document required build tool versions

I cannot find the minimum version of Autoconf that Tor should support. This makes it difficult to improve upon the build configuration without accidentally using functionality not supported by older versions of Autoconf.

My suggestions are to

• use AC_PREREQ in configure.ac.
• move AUTOMAKE_OPTIONS from Makefile.am to configure.ac to have all the version configuration in one file.
• document the versions in one of the doc/HACKING files.

I am willing to write a patch with these changes if someone could tell me the minimum Autoconf version Tor should support.