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.

Object.h

@@ -10,6 +10,7 @@ in the source distribution for its full text.
 
 #include "RichString.h"
 #include "XAlloc.h"
+#include "Macros.h"
 
 typedef struct Object_ Object;