package/ola: fix check for google.protobuf

Fix the google.protobuf check in an upstreamable way.

As suggested by Arnout, use an autoconf ac_cv_have_* variable
so we can pass it on the command line. This is generic behaviour
for a lot of autoconf internals, so it makes sense to do it this
way.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>