Incorporate feedback from Graydon.

Namely, switched in many places to using GNU make provided functions
for directory listing and text processing, rather than spawning a
shell process to do that work.

In the process of the revision, learned about Target-specific
variables, which were very applicable to INSTALL_LIB (which, on a
per-recipe basis, was always receiving the same actual arguments for
its first two formal parameters in every invocation).

  http://www.gnu.org/software/make/manual/html_node/Target_002dspecific.html

(We might be able to make use of those in future refactorings.)

----

Also adds a cleanup pass to get-snapshot.py as well, since the same
problem arises when we unpack libraries from the snapshot archive into
a build directory with a prior snapshot's artifacts.  (I put this step
into the python script rather than the makefile because I wanted to
delay the cleanup pass until after we have at least successfully
downloaded the tarball.  That way, if the download fails, you should
not destroy the previous unarchived snapshot libraries and build
products.)