py/objfloat, py/modmath: Ensure M_PI and M_E defined.

In some compliation enviroments (e.g. mbed online compiler) with
strict standards compliance, <math.h> does not define constants such
as M_PI.  Provide fallback definitions of M_E and M_PI where needed.

py/modmath.c

@@ -31,6 +31,11 @@
 
 #include <math.h>
 
+// M_PI is not part of the math.h standard and may not be defined
+#ifndef M_PI
+#define M_PI (3.14159265358979323846)
+#endif
+
 /// \module math - mathematical functions
 ///
 /// The `math` module provides some basic mathematical funtions for