move Storable CAN_FLOCK computation into XS

move Storable CAN_FLOCK computation into XS

At build time, Storable would check if it could use flock based on some
Config values, and generate the final Storable.pm file using tha value.
This was done to avoid needing to load Config at runtime.  This adds
complexity to the build process.

A simpler option is to build the constant in the XS code.  This means
the build process can be entirely standard for XS.

Some slight adjustments to the code are needed to load the XS module at
BEGIN time, so that the contant is available in the rest of the file.