Optionally attach declarations to the global module rather than `mo…

…dule fmt`

This allows coexistence with TUs that use {fmt} through #include without duplicating declarations, definitions, linker symbols, and object code.

src/fmt.cc

@@ -73,6 +73,10 @@ export module fmt;
   export {
 // All library-provided declarations and definitions must be in the module
 // purview to be exported.
+#ifdef FMT_ATTACH_TO_GLOBAL_MODULE
+extern "C++" {
+#endif
+
 #include "fmt/args.h"
 #include "fmt/chrono.h"
 #include "fmt/color.h"
@@ -83,6 +87,10 @@ export module fmt;
 #include "fmt/xchar.h"
 #include "fmt/std.h"
 
+#ifdef FMT_ATTACH_TO_GLOBAL_MODULE
+}
+#endif
+
 // gcc doesn't yet implement private module fragments
 #if !FMT_GCC_VERSION
 module : private;