Fix compilation in Visual C++ 2013.

I had to fix the following things to make Ceres compile in 2013:
 * Not link to 'm' (GNU math library).
 * Excplicitly convert an std::ostream to bool.
 * Include <algorithm> for std::max.

Change-Id: I3ff65413baf8711364360d46dd71fd553fa63e72

examples/pgm_image.h

@@ -197,7 +197,7 @@ bool PGMImage<Real>::WriteToFile(std::string filename) const {
     outputfile << static_cast<int>(data_[i] + 0.5) << ' ';
   }
 
-  return outputfile;  // Returns true/false
+  return bool(outputfile);  // Returns true/false
 }
 
 namespace  {