Support compile-time strings and compile-time format string compilation in module

Make just the necessary parts available for lookup from client context.

src/fmt.cc

@@ -1,4 +1,8 @@
 module;
+#ifndef __cpp_modules
+#  error Module not supported.
+#endif
+
 // put all implementation-provided headers into the global module fragment
 // to prevent attachment to this module
 #if !defined(_CRT_SECURE_NO_WARNINGS) && defined(_MSC_VER)
@@ -76,10 +80,6 @@ export module fmt;
   }                              \
   export {
 
-#if defined(_MSC_FULL_VER) && _MSC_FULL_VER > 192930036
-#define FMT_USE_NONTYPE_TEMPLATE_PARAMETERS 0
-#endif
-
 // all library-provided declarations and definitions
 // must be in the module purview to be exported
 #include "fmt/args.h"