Merge branch 'debug-level-build'

This fixes the build with DEBUG_LEVEL < 4, which was broken when building
from the repository since --enable-warnings was made the default.

Although, most issues only occurred with the level reduced to 0/-1.  And
while removing debug statements at compile time completely is probably
not useful in production, there might be use cases in certain benchmarking
scenarios.  Also, with the recent changes to the controller there should
only rarely be a listener registered at a higher log level so the overhead
for those higher-level DBG statements is minimal.

Anyway, reducing the log level at compile time is a documented feature and
at least DEBUG_LEVEL=3 could be useful to prevent leaking sensitive
information via logs from the outset.  So we should make sure compilation
doesn't fail.