If pkg-config is not used, look for opus.h in <prefix>/include/opus
or (using --with-opus=<opuspath>) <opuspath>/include/opus, not
<prefix>/include or <opuspath>/include.  As before the opus include
directory can still be overridden with --with-opus-includes=<path>,
but this should not be needed if the development package is
installed under <prefix> or the --with-opus directory.