add transition helper to format.h
the transition from v3 to v4 introduced an API change that format.h does
not provide printf functionality. to easi the transition we introduce a
define `FMT_FORMAT_PROVIDE_PRINTF` that will pull in printf.h from format.h
diff --git a/fmt/format.h b/fmt/format.h
index eccefed..d4eeaa6 100644
--- a/fmt/format.h
+++ b/fmt/format.h
@@ -25,6 +25,11 @@
SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
+// transition helper
+#ifdef FMT_FORMAT_PROVIDE_PRINTF
+#include "printf.h"
+#endif
+
#ifndef FMT_FORMAT_H_
#define FMT_FORMAT_H_
