Support for C++14 shared_mutex in libstdc++

[Edward Diener <eldlistmailingz@xxxxxxxxxxxxxx>]

I am looking for a way to check for C++14 support for the shared_mutex 
header in libstdc++ using preprocessor predefined macros. Is there a way 
to do this ? Does checking for some value of __cplusplus >= 201402 do 
this or is there some other check or further check which I must do ?