Make util/make_version more POSIX-compliant

Fixes PrincetonUniversity#702

I've avoided `&>` as its not POSIX-complaint according to [stack
exchange](https://unix.stackexchange.com/a/590707/45323).

I've also taken the opportunity to simplify slightly the logic that
checks for `git`, relying on the return code of `command -v` rather than
testing that the resulting path exists.

Finally, I've removed the `2>/dev/null` redirect on `command -v`; when the
command does not exist, `command -v` is already silent, and I think we
want noise if `command` itself does not exist or is failing strangely
for some reason.

util/make_version

@@ -1,16 +1,15 @@
 #!/usr/bin/env bash
 #usage:  util/make_version Bitsize Compcert-version
 F=veric/version.v
-builtin type -P gdate &> /dev/null
-if [ $? -eq 0 ]; then
+if command -v gdate >/dev/null; then
     DATE=gdate
 else
     DATE=date
 fi
 set -e
 printf >$F 'Require Import ZArith Coq.Strings.String. Open Scope string.\n'
 printf >>$F 'Definition git_rev := "'
-if [ -e "$(command -v git)" ] && [ "$(git rev-parse --is-inside-work-tree 2>/dev/null)" = "true" ]; then
+if command -v git >/dev/null && [ "$(git rev-parse --is-inside-work-tree 2>/dev/null)" = "true" ]; then
   git log -n 1 --pretty=format:"%H" >>$F || true
 fi
 printf >>$F '".\n'