configure.ac: Add support for --enable-debug

This adds an --enable-debug option which turns on the ENABLE_DEBUG macro
if passed to configure. It does not touch the CFLAGS, as we should be
building with -Wall -g anyway, and the level of debugging info needed is
best left for the user. This closes #76.

Reported-by: Viktor Tusa <tusavik@gmail.com>
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>