Fix #3225: Check for old Rust libraries when building and installing.

When building Rust libraries (e.g. librustc, libstd, etc), checks for
and verbosely removes previous build products before invoking rustc.
(Also, when Make variable VERBOSE is defined, it will list all of the
libraries matching the object library's glob after the rustc
invocation has completed.)

When installing Rust libraries, checks for previous libraries in
target install directory, but does not remove them.

The thinking behind these two different modes of operation is that the
installation target, unlike the build tree, is not under the control
of this infrastructure and it is not up to this Makefile to decide if
the previous libraries should be removed.
Fix #3225: 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.)

----

Also reverted whitespace changes to minimize diff.
I plan to put them back in in a dedicated commit elsewhere.
Fix #3225: Incorporate feedback from Graydon.

This is a small follow-up fix to the previous commit: I needed
to quote the right-hand side of the definition for the variable
MATCHES, to handle the case where there are more than one previously
installed libraries in the target directory.
auto merge of #7637 : pnkfelix/rust/fsk-guard-against-stale-libraries-issue3225-safeguarded, r=graydon

When building Rust libraries (e.g. librustc, libstd, etc), checks for
and verbosely removes previous build products before invoking rustc.
(Also, when Make variable VERBOSE is defined, it will list all of the
libraries matching the object library's glob after the rustc
invocation has completed.)

When installing Rust libraries, checks for previous libraries in
target install directory, but does not remove them.

The thinking behind these two different modes of operation is that the
installation target, unlike the build tree, is not under the control
of this infrastructure and it is not up to this Makefile to decide if
the previous libraries should be removed.

Fixes #3225 (at least in terms of mitigating the multiple library
problem by proactively warning the user about it.)