gfcp can be optimized for sparse files

[nixpanic]

With GlusterFS 3.8 we have support for SEEK_DATA and SEEK_HOLE with glfs_lseek(). If you want to prevent a dependency or #ifdef constructs, you can detect working support for SEEK_DATA and SEEK_HOLE similar to the probing that QEMU does (scheduled for inclusion in the next QEMU version).

I'm not sure when I have time to look into this, just reporting it now in case someone else wants to send a patch :-)