Clean vendor files before touching all of them to avoid missing changes

[dylanahsmith]

Problem

@casperisfine discovered that changes that were made to the vendored libmemcached code weren't having any effect. I tracked the problem down to the command to "Touching all files so autoconf doesn't run." That command was touch both source and output files, which would make it seem like the output files are up to date even after changing a source file then running bundle exec rake clean compile.

Solution

Before touching all files, cleanup all the built files. That way this touching all files hack won't cause this problem.

Perhaps for the next major release we could have autoconf be a build dependency so we wouldn't need this touch files hack to avoid running it.

[casperisfine]

I confirm this fixes my problem. thank you!