Calculate block checksums in parallel.

This will calculate checksums of files using n threads where n is the
number of CPUs you have.

It assigns one file per thread, so individual file checksumming is
synchronous.

Signed-off-by: Russell Harmon <eatnumber1@gmail.com>
Merge branch 'threaded' of https://github.com/eatnumber1/duperemove
- Fix build:

glib_CFLAGS wasn't getting picked up, probably because:

"Subsequent assignments or appends to this variable which are not marked
override will be ignored."

to make life easier I just moved the whole bit down to where
we last need to append to CFLAGS.

Signed-off-by: Mark Fasheh <mfasheh@suse.de>