glr.cc: avoid compiler warnings

[Akim Demaille]

commit 05ae33dc320adb2ffaccc7d18e3ef0e353294f3d
Author: Akim Demaille <address@hidden>
Date:   Tue Dec 17 06:37:47 2019 +0100

    glr.cc: avoid compiler warnings
    
        381. types.at:366: testing glr.cc api.value.type={double} ...
        test.cc:207:57: error: "__clang_major__" is not defined, evaluates to 0 
[-Werror=undef]
          207 | #if defined __APPLE__ && YY_CPLUSPLUS < 201103L && 4 <= 
__clang_major__
              |                                                         
^~~~~~~~~~~~~~~
    
    * data/skeletons/glr.cc: Check __clang_major__ before using it.

diff --git a/data/skeletons/glr.cc b/data/skeletons/glr.cc
index 343b52ef..4dd82970 100644
--- a/data/skeletons/glr.cc
+++ b/data/skeletons/glr.cc
@@ -267,7 +267,8 @@ b4_percent_code_get([[requires]])[
 
 // On MacOS, PTRDIFF_MAX is defined as long long, which Clang's
 // -pedantic reports as being a C++11 extension.
-#if defined __APPLE__ && YY_CPLUSPLUS < 201103L && 4 <= __clang_major__
+#if defined __APPLE__ && YY_CPLUSPLUS < 201103L \
+    && defined __clang__ && 4 <= __clang_major__
 # pragma clang diagnostic ignored "-Wc++11-long-long"
 #endif