maint: build: fix compatibility with old compilers

[Akim Demaille]

commit 6e89bc0fd27ee8bf1095544504c6970530e9527c
Author: Akim Demaille <address@hidden>
Date:   Thu Apr 2 07:09:27 2020 +0200

    build: fix compatibility with old compilers
    
    GCC 4.2 dies with
    
        src/InadequacyList.c: In function 'InadequacyList__new_conflict':
        src/InadequacyList.c:37: error: #pragma GCC diagnostic not allowed 
inside functions
        src/InadequacyList.c:37: error: #pragma GCC diagnostic not allowed 
inside functions
        src/InadequacyList.c:40: error: #pragma GCC diagnostic not allowed 
inside functions
    
    Reported by Evan Lavelle.
    See https://lists.gnu.org/r/bug-bison/2020-03/msg00021.html
    and https://trac.macports.org/ticket/59927.
    
    * src/system.h (GCC_VERSION): New.
    Use it to control IGNORE_TYPE_LIMITS_BEGIN and
    IGNORE_TYPE_LIMITS_END.

diff --git a/THANKS b/THANKS
index d8ef2c0c..9a06506b 100644
--- a/THANKS
+++ b/THANKS
@@ -61,6 +61,7 @@ Enrico Scholz             address@hidden
 Eric Blake                address@hidden
 Eric S. Raymond           address@hidden
 Étienne Renault           address@hidden
+Evan Lavelle              address@hidden
 Evan Nemerson             address@hidden
 Evgeny Stambulchik        address@hidden
 Fabrice Bauzac            address@hidden
diff --git a/src/system.h b/src/system.h
index 0210f6c6..6073048f 100644
--- a/src/system.h
+++ b/src/system.h
@@ -73,9 +73,14 @@ typedef size_t uintptr_t;
 # include <verify.h>
 # include <xalloc.h>
 
-
-/* See https://lists.gnu.org/archive/html/bug-bison/2019-10/msg00061.html. */
-# if defined __GNUC__ && ! defined __clang__ && ! defined __ICC && __GNUC__ < 5
+// Clang and ICC like to pretend they are GCC.
+#if defined __GNUC__ && !defined __clang__ && !defined __ICC
+# define GCC_VERSION (__GNUC__ * 100 + __GNUC_MINOR__)
+#endif
+
+// See https://lists.gnu.org/archive/html/bug-bison/2019-10/msg00061.html
+// and https://trac.macports.org/ticket/59927.
+#if defined GCC_VERSION && 405 <= GCC_VERSION
 #  define IGNORE_TYPE_LIMITS_BEGIN \
      _Pragma ("GCC diagnostic push") \
      _Pragma ("GCC diagnostic ignored \"-Wtype-limits\"")