Fix compile warning, MIN/MAX macros may already be defined by param.h

ext/gd/libgd/gdft.c

@@ -86,8 +86,13 @@ gdImageStringFT (gdImage * im, int *brect, int fg, char *fontlist,
 #define TRUE !FALSE
 #endif
 
+#ifndef MAX
 #define MAX(a,b) ((a)>(b)?(a):(b))
+#endif
+
+#ifndef MIN
 #define MIN(a,b) ((a)<(b)?(a):(b))
+#endif
 
 typedef struct
 {