Fix warnings on clang 3.9

[lgritz]

I guess we extensively use constructs like this:

#if SOMETIMES_DEFINED > 3

as shorthand for

#if defined(SOMETIMES_DEFINED) && SOMETIMES_DEFINED > 3

I was never 100% sure that the former was legal, but the compilers didn't
complain and I preferred it over the more verbose equivalent.

But apparently, it's just lax compilers, and in the latest Clang 3.9,
doing this (assuming "expansion to defined") is a warning, and of course
we usually treat warnings as errors.

It's going to be a lot of work to track all of these down and fix them,
especially before clang 3.9 has a Homebrew package (without it, I'm
operating blind, both on my laptop and on the TravisCI builds). So for
now, we'll settle for just disabling that warning, and slowly fix up the
guards later.

[lgritz]

Ashley Retallack says this fixes the problem (which he reported).
I'll merge as soon as the CI tests all come back.

[lgritz]

Merged.