C: fix deps_include_system semantics.
C: fix deps_include_system semantics.

Turns out I had the -MD/-MMD switches backwards, so this has actually
been *on* by default, contrary to the docs. Because (1) "on" seems like
the right setting, and (2) it hasn't caused any problems, contrary to my
assertion from 2014, I'm going to leave it that way and fix the docs.

cbiffle/cobble-build,cbiffle/cobble-build