build: update autotools scripts
 * add -Wextra to CFLAGS
 * let ./configure take care of debug/release targets

Signed-off-by: Pawel Dziepak <pdziepak@quarnos.org>
Fix warnings reported by -Wextra
 * Mostly unused arguments and signed/unsigned comparison.

Signed-off-by: Pawel Dziepak <pdziepak@quarnos.org>