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.

This unveiled clashes with name 'vfprintf' from `stdio.h`. Drive-by fix using qualified name lookup.

include/fmt/printf.h

@@ -599,7 +599,7 @@ inline auto vsprintf(
     basic_format_args<basic_printf_context_t<type_identity_t<Char>>> args)
     -> std::basic_string<Char> {
   auto buf = basic_memory_buffer<Char>();
-  vprintf(buf, detail::to_string_view(fmt), args);
+  detail::vprintf(buf, detail::to_string_view(fmt), args);
   return to_string(buf);
 }
 
@@ -626,7 +626,7 @@ inline auto vfprintf(
     basic_format_args<basic_printf_context_t<type_identity_t<Char>>> args)
     -> int {
   auto buf = basic_memory_buffer<Char>();
-  vprintf(buf, detail::to_string_view(fmt), args);
+  detail::vprintf(buf, detail::to_string_view(fmt), args);
   size_t size = buf.size();
   return std::fwrite(buf.data(), sizeof(Char), size, f) < size
              ? -1

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;