This patch does two things:

a) It finally implements locking/unlocking with -fcoarray=lib. Before it 
was effectively a no op operation always succeeding. - The 
implementation only works for "normal" locking variables - and neither 
for CLASS, type extension or lock components of derived types.

b) It adds a compile-time error for the cases not supported, avoiding 
race conditions due to an only apparently working locking (as it would 
be the case with the implementation before the patch).

Build and regtested on x86-64-gnu-linux.
OK for the GCC 5 trunk?

Tobias