remove std:: namespace from copysignf and copysign

Aleksandar's recommendation for an Android compile error fix ended up fixing the same compile error for iPhone and iPhonesimulator:

"copysign* is C99/C++ TR1, apparently still not in std namespace with
gcc 4.7. Remove std:: in front of it and see if it helps. If you
provide a patch, we'll test and include it for future releases."