python: Prevent potential build failure

that is caused by not calling the right (newly built)
python binary.

Fix #20245 hopefully.
mercurial: Install python bindings properly

Also allow to build with --HEAD by making brew's
download strategy aware of a brewed but unlined hg.

Adapted the caveats so that the procedure to brew
hg --HEAD is clear.

Mention how to config hgk in the caveats.
Python: Don't use named tuples to get the version

sys.version_info.major is only possible on 2.7+ or so.
Therefore, we now use [0] instead.
Extract attr_rw from Formula for reuse

Closes #20239.
lzlib 1.4

Closes #20253.
Remove postgresql and mysql audit noise

Closes #20159.
Nginx devel 1.5.1

Closes #20247.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
weechat: add git HEAD

Closes #20250.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
latex2html: tex is recommended

Closes #20237.
ZNC does not depend on c-ares

Closes #20257.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
mapnik: remove unused opt_prefix
Mapnik 2.2.0

Closes #20238.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>