We now use `$@` rather than `$*` so that we display the output target
rather than the stem of the rule.  This is more consistent for rules
that users add (where the pattern variable might be something
insufficiently identifying), and also generalizes more nicely to mixing
multiple compilers (we get a clear difference between producing OCaml
files and producing .vo files, even if the filename is the same up to
the suffix).  The result is that it's easy to describe what the timing
information of the build log records: each time comes with a label which
is a file name, and the time is the time it takes to produce that file.

We fix
```
Makefile.build:222: warning: undefined variable '*'
```
by not passing a time format including a Makefile variable when not
inside a target (and whether or not the command succeeds should not
depend on the particular format in any case, and all we are testing for
is whether or not the command exists and supports `-f`).

This commit adds timing scripts from
https://github.com/JasonGross/coq-scripts/tree/master/timing into the
tools folder, and integrates them into coq_makefile and Coq's makefile.
The main added makefile targets are:

- the `TIMING` variable - when non-empty, this creates for each built
  `.v` file a `.v.timing` variable (or `.v.before-timing` or
  `.v.after-timing` for `TIMING=before` and `TIMING=after`, respectively)
- `pretty-timed TGTS=...` - runs `make $(TGTS)` and prints a table of
  sorted timings at the end, saving it to `time-of-build-pretty.log`
- `make-pretty-timed-before TGTS=...`, `make-pretty-timed-after
  TGTS=...` - runs `make $(TGTS)`, and saves the timing data to the file
  `time-of-build-before.log` or `time-of-build-after.log`, respectively
- `print-pretty-timed-diff` - prints a table with the difference between
  the logs recorded by `make-pretty-timed-before` and
  `make-pretty-timed-after`, saving the table to
  `time-of-build-both.log`
- `print-pretty-single-time-diff BEFORE=... AFTER=...` - this prints a
  table with the differences between two `.v.timing` files, and saves
  the output to `time-of-build-pretty.log`
- `*.v.timing.diff` - this saves the result of
  `print-pretty-single-time-diff` for each target to the
  `.v.timing.diff` file
- `all.timing.diff` (`world.timing.diff` and `coq.timing.diff` in Coq's
  own Makefile) - makes all `*.v.timing.diff` targets

N.B. We need to make `make pretty-timed` fail if `make` fails.  To do
this, we need to get around the fact that pipes swallow exit codes.
There are a few solutions in
https://stackoverflow.com/questions/23079651/equivalent-of-pipefail-in-gnu-make;
we choose the temporary file rather than requiring the shell of the
makefile to be bash.