Windows doesn't have M_PI; define it ourselves when needed.

This should fix the Windows build, broken by commit ed802e7.

Author: hlinnaka

contrib/pgbench/pgbench.c

@@ -53,6 +53,10 @@
 #define INT64_MAX	INT64CONST(0x7FFFFFFFFFFFFFFF)
 #endif
 
+#ifndef M_PI
+#define M_PI 3.14159265358979323846
+#endif
+
 /*
  * Multi-platform pthread implementations
  */