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.

CPUMeter.h

@@ -24,13 +24,6 @@ typedef enum {
 
 extern int CPUMeter_attributes[];
 
-#ifndef MIN
-#define MIN(a,b) ((a)<(b)?(a):(b))
-#endif
-#ifndef MAX
-#define MAX(a,b) ((a)>(b)?(a):(b))
-#endif
-
 extern MeterClass CPUMeter_class;
 
 extern MeterClass AllCPUsMeter_class;