autoconf: remove get_command_descriptions  dependency to gtest

It is not required and a leftover of the time when it was a unit test
which was a mistake to begin with. It has been reported to create
dependencies problems in some cases, when gest is not available at the
time get_command_descriptions is compiled.

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #636 from dachary/wip-get_command_description

autoconf: remove get_command_descriptions  dependency to gtest

Reviewed-by: Sage Weil <sage@inktank.com>