Consolidate repeated macro definitions into one header

The MIN, MAX, CLAMP, MINIMUM, and MAXIMUM macros appear
throughout the codebase with many re-definitions.  Make
a single copy of each in a common header file, and use
the BSD variants of MINIMUM/MAXIMUM due to conflicts in
the system <sys/param.h> headers.

RichString.c

@@ -7,17 +7,13 @@ in the source distribution for its full text.
 
 #include "RichString.h"
 #include "XAlloc.h"
+#include "Macros.h"
 
 #include <stdlib.h>
 #include <string.h>
 
 #define RICHSTRING_MAXLEN 350
 
-
-#ifndef CLAMP
-#define CLAMP(x,low,high) (((x)>(high))?(high):(((x)<(low))?(low):(x)))
-#endif
-
 #define charBytes(n) (sizeof(CharType) * (n))
 
 static void RichString_extendLen(RichString* this, int len) {