Enable support for silent-rules

When building interactivly, the error ouptut is nice to have visible, without really long lines, so errors can be found easier in the logs. Non-interactive builds should be verbose however, like travis-builds, so we can easily see the reason of the errors there.

Earlier, configure.ac didn't support silent rules, so just enable the support, but don't activate it.

Signed-off-by: Max Sikstrom <max.sikstrom@op5.com>