Advise porter on where to make necessary change.

In swiftlang#1075 the change was already made for BSD (thank you!); my working
edit had this guidance to ensure future porters get an error directing
them where to make a necessary change.

Otherwise, the FoundationEssentials build will fail and complain these
variables are not defined but not have guidance as to where they are
sourced from.

Author: 3405691582

Sources/_FoundationCShims/include/_CStdlib.h

@@ -156,6 +156,8 @@
 #ifndef TZDEFAULT
 #define TZDEFAULT    "/etc/localtime"
 #endif /* !defined TZDEFAULT */
+#else
+#error "define TZDIR and TZDEFAULT for this platform
 #endif /* TARGET_OS_MAC || TARGET_OS_LINUX || TARGET_OS_BSD */
 
 #endif