Change configure to set debugging to disabled by default

This changes the default in configure to disable debugging, the result is that the we compile with "-O2" instead of "-DDEBUG -fno-omit-frame-pointer" as default. Given that the warnings threated as errors in #33 has not been found earlier I can only conclude that SOGo is never build with --disable-debug and was thus always compiled without optimization (unless you override OBJCFLAGS when calling make such as I do in my Debian packaging). I think this shows that building with optimization should really be the default.

Of course #33 needs to be applied before this PR or else the OpenChange backend won't compile.