[PATCH 3 of 3 bsdmake stable?] Makefile: use shell-command assignment instead of $(eval ...)

[Sean Farley]

Augie Fackler <raf at durin42.com> writes:

> # HG changeset patch
> # User Augie Fackler <augie at google.com>
> # Date 1461247880 14400
> #      Thu Apr 21 10:11:20 2016 -0400
> # Branch stable
> # Node ID 9471e80ea13cefc02a4567a26d17914c8d7237b6
> # Parent  d798163ea54318407c4a86ae65ec94774a19eb0a
> Makefile: use shell-command assignment instead of $(eval ...)
>
> This is portable between BSD and GNU make.
>
> As of this change, our Makefile appears to work in both BSD and GNU
> make, with the caveat that the test-% and testpy-% wildcard rules
> don't work on BSD make. That said, this still seems worthwhile because
> it lets the buildbots work more consistently across platforms.

I would lean towards applying this so our CI builds will work with 3.8
but I defer to The Old Ones.