gcc-4.8 introduced a new optimization level -Og which enables
optimizations that do not interfere with debugging.

When configured with --enable-debug, use -Og rather than -O0
if available.

Change-Id: Iff3f98d736681ae34e49b96510228a14ce456b34
Reviewed-on: https://gerrit.libreoffice.org/31333Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>