Where is C++'s math.h's double abs(double)?

[Jeffrey Oldham]

Why is C++ program that includes math.h not able to access
			 double abs (double)
?  libstdc++-v3/include/c_shadow/math.h seems to be the right file,
but it is not installed.  libstdc++-v3/include/c_shadow/string.h also
seems not to be installed.

Thanks,
Jeffrey D. Oldham
oldham@codesourcery.com
#include <iostream>
#include <math.h>

int main()
{
  std::cout << abs (-1.23e+3) << std::endl;
  return 0;
}