fix --enable-debug=no and --disable-debug to actually disable debug

Someone using our configure script found that when you pass --disable-debug or --enable-debug=no on the command line for configure was always getting debug enabled. The macro AC_ARG_ENABLE action-if-found was being treated as action-if-enabled. The old, old debug.m4 macro was doing it this way, so I massaged this macro to behave the same way.