[pdiff] Don't use float math functions if not using gcc with C99

The float version of many math functions were introduced in C99, and were
causing compile failure on systems like OS X.  We now define them to their
double variant if __USE_ISOC99 is not defined.  We may want to expand it later
to cover non-gcc compilers too, but since this is pdiff only, it's not really
important.

test/pdiff/pdiff.c

@@ -25,6 +25,14 @@
 #define M_PI 3.14159265f
 #endif
 
+#ifndef __USE_ISOC99
+#define expf	exp
+#define powf	pow
+#define fabsf	fabs
+#define sqrtf	sqrt
+#define log10f	log10
+#endif
+
 /*
  * Given the adaptation luminance, this function returns the
  * threshold of visibility in cd per m^2