build: work around warnings in Flex 2.5.35

[Akim Demaille]

In master.

 commit ea0db44fedc8d5cbdc5c3180bef0285d7ae83803
 Author: Akim Demaille <address@hidden>
 Date:   Sat Sep 8 06:47:08 2018 +0200

    build: work around warnings in Flex 2.5.35
    
    That's the version on Ubuntu Precise.
    See also 1dac131ec45ffa1e382319a94640c65bd10f6aa5.
    
    * src/flex-scanner.h: Disable -Wdocumentation.
    * doc/bison.texi: Turn off a warning triggered by Flex 2.6.4.

diff --git a/doc/bison.texi b/doc/bison.texi
index b7945343..55e2b46f 100644
--- a/doc/bison.texi
+++ b/doc/bison.texi
@@ -11426,8 +11426,8 @@ parser's to get the set of defined tokens.
 # define yywrap() 1
 
 // Pacify warnings in yy_init_buffer (observed with Flex 2.6.4)
-// and GCC 7.3.0.
-#if defined __GNUC__ && 7 <= __GNUC__
+// and GCC 6.4.0, 7.3.0.
+#if defined __GNUC__ && !defined __clang__ && 6 <= __GNUC__
 # pragma GCC diagnostic ignored "-Wnull-dereference"
 #endif
 address@hidden
diff --git a/src/flex-scanner.h b/src/flex-scanner.h
index ac3b7338..66b6098e 100644
--- a/src/flex-scanner.h
+++ b/src/flex-scanner.h
@@ -27,6 +27,32 @@
    + (YY_FLEX_MINOR_VERSION) * 1000     \
    + (YY_FLEX_SUBMINOR_VERSION))
 
+// Pacify warnings in yy_init_buffer (observed with Flex 2.6.4 and GCC
+// 6.4.0 and 7.3.0).
+//
+// ./src/scan-skel.c: In function 'skel_restart':
+// ./src/scan-skel.c:2035:20: error: potential null pointer dereference 
[-Werror=null-dereference]
+//   b->yy_fill_buffer = 1;
+//   ~~~~~~~~~~~~~~~~~~^~~
+// ./src/scan-skel.c:2031:19: error: potential null pointer dereference 
[-Werror=null-dereference]
+//   b->yy_input_file = file;
+//   ~~~~~~~~~~~~~~~~~^~~~~~
+#if defined __GNUC__ && ! defined __clang__ && 6 <= __GNUC__
+# pragma GCC diagnostic ignored "-Wnull-dereference"
+#endif
+
+// Old versions of Flex (2.5.35) generate an incomplete documentation comment.
+//
+//  In file included from src/scan-code-c.c:3:
+//  src/scan-code.c:2198:21: error: empty paragraph passed to '@param' command
+//        [-Werror,-Wdocumentation]
+//   * @param line_number
+//     ~~~~~~~~~~~~~~~~~^
+//  1 error generated.
+#if FLEX_VERSION <= 20060000 && defined __clang__
+# pragma clang diagnostic ignored "-Wdocumentation"
+#endif
+
 /* Pacify "gcc -Wmissing-prototypes" when flex 2.5.31 is used.  */
 #if FLEX_VERSION <= 2005031
 int   FLEX_PREFIX (get_lineno) (void);
@@ -44,20 +70,6 @@ int   FLEX_PREFIX (lex_destroy) (void);
 
 #define last_string    FLEX_PREFIX (last_string)
 
-// Pacify warnings in yy_init_buffer (observed with Flex 2.6.4 and GCC
-// 6.4.0 and 7.3.0).
-//
-// ./src/scan-skel.c: In function 'skel_restart':
-// ./src/scan-skel.c:2035:20: error: potential null pointer dereference 
[-Werror=null-dereference]
-//   b->yy_fill_buffer = 1;
-//   ~~~~~~~~~~~~~~~~~~^~~
-// ./src/scan-skel.c:2031:19: error: potential null pointer dereference 
[-Werror=null-dereference]
-//   b->yy_input_file = file;
-//   ~~~~~~~~~~~~~~~~~^~~~~~
-#if defined __GNUC__ && 6 <= __GNUC__
-# pragma GCC diagnostic ignored "-Wnull-dereference"
-#endif
-
 /* It seems to be a nice "feature" of Flex that one cannot use yytext,
    yyleng etc. when a prefix is given, since there is no longer a
    #define, but rather the token is actually changed in the output.