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 This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a