Turns out stlport isn't broken. <cmath> (included
transitively via gtest in our case) is not required
to make C99 math macros (like signbit) available, nor is
it required to preserve them if they're already defined.
It is only required to make the equivalent functions in
namespace std available.
I couldn't find any documentation of required behaviour for
programs that include both <math.h> and <cmath>.
I've verified experimentally that llvm's libc++ and gnu
stl behave the same as stlport.
bug: 12935307
Change-Id: I9dc5cc0fd9f4f259abc8eefb280177cdd092a94b
5f6f4a956c