Merge pull request #348 from LogicalKnight/fix-clang-extern-header-only

Fix building with clang in header-only mode
diff --git a/fmt/format.h b/fmt/format.h
index 2c0b36e..bcae5a2 100644
--- a/fmt/format.h
+++ b/fmt/format.h
@@ -841,7 +841,7 @@
 # define FMT_USE_EXTERN_TEMPLATES (__clang__ && FMT_USE_VARIADIC_TEMPLATES)
 #endif
 
-#if FMT_USE_EXTERN_TEMPLATES
+#if FMT_USE_EXTERN_TEMPLATES && !defined(FMT_HEADER_ONLY)
 extern template struct BasicData<void>;
 #endif