Cycles: Fix compilation error on with older GCC

Hopefully it works on all platforms now.

intern/cycles/util/util_math.h

@@ -22,6 +22,11 @@
  * Basic math functions on scalar and vector types. This header is used by
  * both the kernel code when compiled as C++, and other C++ non-kernel code. */
 
+#ifndef __KERNEL_GPU__
+#  include <cmath>
+#endif
+
+
 #ifndef __KERNEL_OPENCL__
 
 #include <float.h>
@@ -97,6 +102,9 @@ ccl_device_inline float fminf(float a, float b)
 
 #ifndef __KERNEL_GPU__
 
+using std::isfinite;
+using std::isnan;
+
 ccl_device_inline int abs(int x)
 {
 	return (x > 0)? x: -x;