Add LDAP_CONST macro for use within EXTERNAL headers such as

<lber.h> and <ldap.h>.
Note: This should not be used within the implementation itself as
our "portable.h" mechanism provides Standard C vs. K&R "const" handling.

include/ldap_cdefs.h

@@ -31,6 +31,10 @@
 #	define LDAP_STRING(x)	#x /* stringify without expanding x */
 #	define LDAP_XSTRING(x)	LDAP_STRING(x) /* expand x, then stringify */
 
+#ifndef LDAP_CONST
+#	define LDAP_CONST	const
+#endif
+
 #else /* no prototypes */
 
 	/* traditional C */
@@ -38,6 +42,10 @@
 #	define LDAP_CONCAT(x,y)	x/**/y
 #	define LDAP_STRING(x)	"x"
 
+#ifndef LDAP_CONST
+#	define LDAP_CONST	/* no const */
+#endif
+
 #endif /* no prototypes */