add -dead_strip_dylibs by default to the macOS linker

Under macOS, to reduce dependency chains, we remove functions and
data that are unreachable by the entry point or exported symbols.
In particular, the linker is run with "-dead_strip" and with
"-dead_strip_dylibs".

custom.h

@@ -73,6 +73,7 @@ struct custom {
  *
  * These are the required interfaces.  The dummy.c stubs these interfaces too.
  */
+EXTERN int custtbl_allowed;		/* 1 ==> CUSTOM defined, 0 ==> CUSTOM undefined */
 E_FUNC VALUE custom(char*, int, VALUE**);	/* master custom interface */
 EXTERN BOOL allow_custom;		/* TRUE => custom builtins allowed */
 E_FUNC void showcustom(void);		/* print custom functions */