Fix warnings on C++98

Author: vitaut

fmt/format.h

@@ -261,7 +261,7 @@ typedef __int64          intmax_t;
 // For GCC according to cppreference.com they were introduced in 3.3.
 # define FMT_USE_EXTERN_TEMPLATES \
     ((__clang__ && FMT_USE_VARIADIC_TEMPLATES) || \
-     FMT_GCC_VERSION >= 303)
+     (FMT_GCC_VERSION >= 303 && FMT_HAS_GXX_CXX11))
 #endif
 
 #ifdef FMT_HEADER_ONLY