Activate extra compiler checks (#667)

Activate extra compiler checks (#667)

If the compiler supports it, add the `-Wextra` compiler flag to enable
extra checks. Do not enable extra checks with GCC v4 because that old
GCC version reports too many false positives.

Explicitly disable those `-Wextra` checks that offer few insights while
creating a lot of noise which cannot be easily avoided.

Fix warnings reported by GCC or clang with the new settings.

Rework syslog-related code in debug.cc, making it clearer and with less
dependencies on some syslog levels.