tree package build errors ignored

[joachifm]

The tree package fails to build but the errors are being ignored, resulting
in a closure without the actual tree binary.
The problem appears to be that $CC is unset and that its arguments
are passed directly to the command interpreter.

Pasting from http://hydra.nixos.org/build/17496175/log/raw for reference:

ggdb -Wall -DLINUX -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -c -o tree.o tree.c
/nix/store/bzqcs6b5r64xf4v86m2a5pcndhv1lnab-bash-4.3-p30/bin/bash: ggdb: command not found
make: [tree.o] Error 127 (ignored)
building unix.o
[...]
o tree  tree.o unix.o html.o xml.o json.o hash.o color.o
/nix/store/bzqcs6b5r64xf4v86m2a5pcndhv1lnab-bash-4.3-p30/bin/bash: o: command not found
make: [tree] Error 127 (ignored)
building install