The libc++ defines this (wrapped in ifndef, like this should have
been), so including a libc++ header and then defining this results in
a werror failure. Define this in common.mk so we can be sure it does
get defined, but also that it gets defined early enough that anyone
defining this in source wrapped with ifndef doesn't get broken.
Test: mma # on angler
Bug: http://b/34740564
Change-Id: Id7d70a6ad5e530390eb8b37e5e618312d1d7e6dc
789f042d3c