[Patch, Fortran] add co_reduce to libcaf_single

[Tobias Burnus]

An early PING.

Last year, Tobias Burnus wrote:
> Looking through the stashed patches, I realized a pending 
> (unsubmitted) patch, showing that both a test case for CO_REDUCE was 
> missing and that libcaf_single didn't include co_reduce.
>
> This patch adds them.
>
> Build and regtested on x86-64-gnu-linux.
> OK for the trunk?
>
> [Still to do for co_reduce: Checking whether any recent changes to the 
> coarray draft requires changes to that feature. I think there were 
> changes to co_reduce.]
>
> Tobias
>
> PS: For GCC 5, I really should complete the coarray support: add 
> LOCKING as missing F2008 feature (basic support only; currently its 
> pretending to always succeed), catch at compile time unsupported 
> features (rather than to fail at run time with wrong results) plus 
> adding vector support in libcaf_single.