Revert "Allow --with-bonjour to work with non-macOS implementations of Bonjour."

Upon further review, our Bonjour code doesn't actually work with the
Avahi not-too-compatible compatibility library.  While you can get it
to work on non-macOS platforms if you link to Apple's own mDNSResponder
code, there don't seem to be many people who care about that.  Leaving in
the AC_SEARCH_LIBS call seems more likely to encourage people to build
broken configurations than to do anything very useful.

Hence, remove the AC_SEARCH_LIBS call and put in a warning comment instead.

Discussion: https://postgr.es/m/2D8331C5-D64F-44C1-8717-63EDC6EAF7EB@brightforge.com