build: Include flac afh only if libogg is available.

build: Include flac afh only if libogg is available.

In configure.ac we have

	AC_DEFUN([NEED_FLAC_OBJECTS], [{
		test "$HAVE_OGG" = 'yes' -a "$HAVE_FLAC" = 'yes'
	}])

In particular, we do not include the flac audio format handler on
systems where libflac is installed but libogg is not. In afh_common.c,
however, we only check for HAVE_FLAC. This results in a build failure
on those systems.

Fix this by modifying the check in afh_common.c to match the check
in configure.ac.