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/custtbl.c

@@ -1,7 +1,7 @@
 /*
  * custtbl - custom interface table
  *
- * Copyright (C) 1999-2007,2021  Landon Curt Noll
+ * Copyright (C) 1999-2007,2021,2023  Landon Curt Noll
  *
  * Calc is open software; you can redistribute it and/or modify it under
  * the terms of the version 2.1 of the GNU Lesser General Public License
@@ -24,6 +24,18 @@
  * Share and enjoy!  :-)	http://www.isthe.com/chongo/tech/comp/calc/
  */
 
+
+/*
+ * ISO C requires a translation unit to contain at least one declaration,
+ * so we declare a global variable whose value is based on if CUSTOM is defined.
+ */
+#if defined(CUSTOM)
+int custtbl_allowed = 1;	/* CUSTOM defined */
+#else /* CUSTOM */
+int custtbl_allowed = 0;	/* CUSTOM undefined */
+#endif /* CUSTOM */
+
+
 #include <unistd.h>
 
 #include "../have_const.h"
@@ -35,8 +47,7 @@
 
 
 /*
- * NOTE: See the file CUSTOM for instructions on how to add
- *	 custom functions.
+ * NOTE: See the file HOW_TO_ADD for instructions on how to add custom functions.
  */