doc: work around Flex's use of 'register'

[Akim Demaille]

commit 154bc7ddacb3d1ad495a7ecda57fcbfaeb6d4f07
Author: Akim Demaille <akim.demai...@gmail.com>
Date:   Sat Sep 22 13:57:48 2018 +0200

    doc: work around Flex's use of 'register'
    
    The CI uses an old version of Flex.
    See 65fa634cdcfc5cf59b8b074670f488bba4df57cd.
    
    * doc/bison.texi (calc++/scanner.ll): Here.

diff --git a/doc/bison.texi b/doc/bison.texi
index 3726854c..e0364b7e 100644
--- a/doc/bison.texi
+++ b/doc/bison.texi
@@ -11506,9 +11506,13 @@ then the parser's to get the set of defined tokens.
 #endif
 
 // Old versions of Flex (2.5.35) use 'register'.  Warnings introduced in
-// GCC 7.
-#if FLEX_VERSION < 206 && defined __GNUC__ && !defined __clang__ && 7 <= 
__GNUC__
-# pragma GCC diagnostic ignored "-Wregister"
+// GCC 7 and Clang 6.
+#if FLEX_VERSION < 206
+# if defined __clang__ && 6 <= __clang_major__
+#  pragma clang diagnostic ignored "-Wdeprecated-register"
+# elif defined __GNUC__ && 7 <= __GNUC__
+#  pragma GCC diagnostic ignored "-Wregister"
+# endif
 #endif
 %@}
 @end example