Define EPSILONs only if not already defined

Fixes OSX buildbot

src/doomdef.h

@@ -411,8 +411,13 @@ INT32 I_GetKey(void);
 #endif
 
 // Floating point comparison epsilons from float.h
+#ifndef FLT_EPSILON
 #define FLT_EPSILON 1.1920928955078125e-7f
+#endif
+
+#ifndef DBL_EPSILON
 #define DBL_EPSILON 2.2204460492503131e-16
+#endif
 
 // An assert-type mechanism.
 #ifdef PARANOIA