Fix more warnings on older GCC.

format.cc

@@ -117,7 +117,7 @@ const char fmt::internal::DIGITS[] =
     "6061626364656667686970717273747576777879"
     "8081828384858687888990919293949596979899";
     
-const uint64_t fmt::internal::POWERS_OF_10[] = {
+FMT_GCC_EXTENSION const uint64_t fmt::internal::POWERS_OF_10[] = {
   0,
   10,
   100,

format.h

@@ -44,6 +44,7 @@
 
 #ifdef __GNUC__
 # define FMT_GCC_VERSION (__GNUC__ * 100 + __GNUC_MINOR__)
+# define FMT_GCC_EXTENSION __extension__
 #endif
 
 // Compatibility with compilers other than clang.
@@ -79,13 +80,8 @@ namespace fmt {
 
 // Fix the warning about long long on older versions of GCC
 // that don't support the diagnostic pragma.
-#ifdef __GNUC__
-__extension__ typedef long long LongLong;
-__extension__ typedef unsigned long long ULongLong;
-#else
-typedef long long LongLong;
-typedef unsigned long long ULongLong;
-#endif
+FMT_GCC_EXTENSION typedef long long LongLong;
+FMT_GCC_EXTENSION typedef unsigned long long ULongLong;
 
 namespace internal {