Workaround ABI compatibility between clang and gcc

Author: vitaut

include/fmt/base.h

@@ -923,7 +923,7 @@ class locale_ref {
  public:
   constexpr locale_ref() : locale_(nullptr) {}
 
-  template <typename Locale, FMT_ENABLE_IF(sizeof(Locale::collate) != 0)>
+  template <typename Locale, int = Locale::collate>
   locale_ref(const Locale& loc) : locale_(&loc) {
     // Check if std::isalpha is found via ADL to reduce the chance of misuse.
     isalpha('x', loc);