osx: delay version computation on macOS builds
Before this patch, HGVER would be evaluated at the beginning of the make
execution, and would be unset because build/mercurial/ doesn't exist yet
at that point. Now we compute the version after the `make install` run
has completed.
This is backported to stable from
8626b44516c1, but that revision had an
error in the shell invocation syntax.
$ hg init
$ echo 123 > a
$ hg add a
$ hg commit -m "first" a
$ mkdir sub
$ echo 321 > sub/b
$ hg add sub/b
$ hg commit -m "second" sub/b
$ cat sub/b
321
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ cat sub/b 2>/dev/null || echo "sub/b not present"
sub/b not present
$ test -d sub || echo "sub not present"
sub not present