Bug#34309: '_PC' macro redefinition

For reasons that are now a mystery, we had defined a CPP symbol to
help ancient compilers work better (in some way that's lost to history).
This interferes with at least one modern compiler.

Now, don't define the _PC symbol.  Those other underscore-leading
symbols are suspect also, but at least the names aren't inscrutable.
Let's leave them for now.

include/my_global.h

@@ -621,7 +621,6 @@ C_MODE_END
 */
 #define _VARARGS(X) X
 #define _STATIC_VARARGS(X) X
-#define _PC(X)	X
 
 /* The DBUG_ON flag always takes precedence over default DBUG_OFF */
 #if defined(DBUG_ON) && defined(DBUG_OFF)