Only build libsrtp tests if a build flag is set

After the most recent setback (Android targets break if you change
their output_dir) I've decided enumerating the configurations where
these tests can build isn't worth the effort. They're not run, anyway.

Let clients ask for them if they want them.

BUG=webrtc:6376
R=kjellander@chromium.org

Review URL: https://codereview.chromium.org/2433343002 .
1 file changed