Guix,

This improves the warning given when free space looks sus.

From:

  note: build failure may have been caused by lack of free disk 
  space

to:

  note: only 0.01 MiB available in ‘/gnu/store’
  note: only 5.00 MiB available in ‘/tmp/guix-build-foo.drv-0’
  note: build failure may have been caused by lack of free disk 
  space

It also raises the warning threshold from 8 to 64 MiB, which is a 
much prettier arbitrary integer.

Question: shouldn't all of nix/ have licence headers added too? 
As it stands, there are two very lonely ones in 
nix/libstore/builtins.{cc,hh} and that's it.

Kind regards,

T G-R