fix API build for C apps

-  protect bool variable in cplusplus in diagnostics.h

lib/diagnostics.h

@@ -34,7 +34,9 @@
 #include <dlfcn.h>
 #endif
 
+#ifdef __cplusplus
 extern bool main_exited;
+#endif
 
 // some of the Android stuff below causes seg faults on some devices.
 // Disable by default.