Comment 2 for bug 1586808

> Changing that cmath to #include "/usr/include/c++/6.1.1/math.h"
did the trick.

That's just a workaround though -- not a real fix.

> We may also move verify that we include util/math.h before all external include files that
use fpclassify and friends.

Correct me if I'm wrong -- but if we include util/math.h and a file we include afterwards includes cmath, then we will see the same problem, right? What about headers outside of our control?