doc: fix garbage text in generated HTML guides

doc: fix garbage text in generated HTML guides

If the user has MAKEFLAGS set in the environment when building the
documentation, the doc/guides/conf.py script which calls "make"
exclusively to get the project version might pick up garbage from
stdout, like:

<title>FAQ &mdash; Data Plane Development Kit make[2]:
Entering directory &#39;/build/1st/dpdk-18.11/doc/guides&#39;
18.11.0
make[2]: Leaving directory &#39;/build/1st/dpdk-18.11&#39;
documentation</title>

Override MAKEFLAGS in the Python subprocess call to avoid this issue.

Fixes: f7aaae2fe6f7 ("doc: add copyright and version")
Cc: stable@dpdk.org
Signed-off-by: Luca Boccassi <bluca@debian.org>