add configure option to disable manpage generation

using --disable-manpage-support it's finally possibly to disable
the formerly obligatory use of a2x to generate the manpage
documentation.

this is the final solution to the decade old problem that users need
to install the enormous asciidoc package to compile TINYproxy from
source, or otherwise get a build error, even though the vast majority
is only interested in the program itself.

solution was inspired by PR #179.

closes #179
closes #111

note that since 1.10.0 release the generated release tarball includes
the generated manpages too; in which case neither the use of a2x
nor --disable-manpage-support is required.

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