build: Combine CFLAGS and DEBUG_CFLAGS.

We needed two sets of flags for gengetopt because the C code generated
by gengetopt would not compile cleanly with our rather strict set of
flags. With lopsub this is no longer necessary.

Moreover, it is considered good practice to not set CFLAGS at all but
to append the contents of this variable to the compile command. This
way the user may set the variable to override some of the options.

This commit gets rid of CFLAGS in favor of DSS_CFLAGS, which is just
the union of the CFLAGS and the DEBUG_CFLAGS variables we had before.