setup: build C extensions with -Werror=declaration-after-statement
MSVC 2008 still needs declarations at the top of the scope. I added it to the
3rd party code too in case somebody vendors a new version with a problem-
they'll get an early warning. Clang seems to ignore this (at least on 10.14
with Xcode 10), and gcc 7.4 will error out as desired on Ubuntu 18.04. Thanks
to Yuya for remembering the name of the option.
Differential Revision: https://phab.mercurial-scm.org/D8318