Autotools: optimize with -Og when debugging

Autotools: optimize with -Og when debugging

When debug (-g) is enabled (default when compiling a development
version), the existing -O optimization level is not correct, since
many variables are optimized out when debugging.

Use -Og instead, and condition not on compiling a development version,
but on --enable-debug. This is the same by default, but it more
precise in our case.