Calculate the value of CFG_FILENAME_EXTRA from the version

[brson]

This value has not changed in a long time, but to support sxs installation in a single dir it needs to. It should just be hashed from the value of CFG_RELEASE.

[lambda-fairy]

@brson So should we pipe the value through e.g. md5sum?

That could be awkward on BSDs, where the hash programs have a different naming convention and interface.

I think we can patch over the differences by adding a check to the configure script. Something like this:

# Use `md5sum` on GNU platforms, and `md5 -q` on BSD
system_md5sum=$(which md5sum)
system_md5=$(which md5)
if [ -n "$system_md5sum" ]
then
    CFG_HASH_COMMAND="$system_md5sum | head -c 8"
elif [ -n "$system_md5" ]
then
    CFG_HASH_COMMAND="$system_md5 -q | head -c 8"
else
    err 'either `md5sum` or `md5` must be installed'
fi
putvar CFG_HASH_COMMAND

then in the Makefile we have:

CFG_FILENAME_EXTRA=$(shell echo -n $(CFG_RELEASE) | $(CFG_HASH_COMMAND))

But if there's a simpler way then I'm all ears.