change macOS to not use "-dead_strip_dylibs" when CUSTOM is defined

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

While calc on macOS will execute if linker used with "-dead_strip_dylibs"
and CUSTOM is defined, other applications that use libcalc but not
libcustcalc (such as sample_many and sample_rand) will fail to execute
due to missile symbols.  Therefore "-dead_strip_dylibs" is not used
by default when ALLOW_CUSTOM is "-DCUSTOM" under macOS.

custom.h

@@ -78,9 +78,6 @@ 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 */
 E_FUNC void customhelp(char *);		/* direct custom help */
-#if defined(CUSTOM)
-EXTERN CONST struct custom cust[];	/* custom interface table */
-#endif /* CUSTOM */
 E_FUNC void init_custreg(void);		/* initialize custom registers */
 
 #endif /* !CUSTOM_H */