doc: -Wzero-as-null-pointer-constant was added to GCC 4.7

[Akim Demaille]

commit f1e0839c9ee6cea5e1722c7ade43439070833ac1
Author: Akim Demaille <address@hidden>
Date:   Sat Nov 3 07:13:19 2018 +0100

    doc: -Wzero-as-null-pointer-constant was added to GCC 4.7
    
    It is not supported by previous versions.
    See https://www.gnu.org/software/gcc/gcc-4.7/changes.html
    Reported by Nelson H. F. Beebe.
    
    * doc/bison.texi (Calc++ Scanner): here.

diff --git a/doc/bison.texi b/doc/bison.texi
index 275332ca..ec78af7e 100644
--- a/doc/bison.texi
+++ b/doc/bison.texi
@@ -11836,7 +11836,7 @@ then the parser's to get the set of defined tokens.
 // Of course, when compiling C as C++, expect warnings about NULL.
 #if defined __clang__
 # pragma clang diagnostic ignored "-Wzero-as-null-pointer-constant"
-#elif defined __GNUC__
+#elif defined __GNUC__ && 407 <= __GNUC__ * 100 + __GNUC_MINOR__
 # pragma GCC diagnostic ignored "-Wzero-as-null-pointer-constant"
 #endif