Add MOJO_{BEGIN,END}_EXTERN_C macros, and use them.

mojo/public/c/system/main.h

Index: mojo/public/c/system/main.h
diff --git a/mojo/public/c/system/main.h b/mojo/public/c/system/main.h
index 5ca6a46fc57fa9c3fb92515a879f6be6c1e00362..9df05ca8e492a79b7bd33bebbc612d4e4d1d3441 100644
--- a/mojo/public/c/system/main.h
+++ b/mojo/public/c/system/main.h
@@ -6,6 +6,7 @@
 #define MOJO_PUBLIC_C_SYSTEM_MAIN_H_
 
 #include "mojo/public/c/system/handle.h"
+#include "mojo/public/c/system/macros.h"
 #include "mojo/public/c/system/result.h"
 
 // Implement MojoMain directly as the entry point for an application.
@@ -14,15 +15,11 @@
 //   ...
 // }
 
-#if defined(__cplusplus)
-extern "C" {
-#endif
+MOJO_BEGIN_EXTERN_C
 
 __attribute__((visibility("default"))) MojoResult MojoMain(
     MojoHandle application_request);
 
-#ifdef __cplusplus
-}  // extern "C"
-#endif
+MOJO_END_EXTERN_C
 
 #endif  // MOJO_PUBLIC_C_SYSTEM_MAIN_H_