add --with-compat by default

[ruslantalpa]

without this flag, building dynamic modules, targeting a specific openresty package release is a pain. One has to use -V option to figure out the build arguments used when building the official release and jsut a copy/pasted does not work since paths are not the same

Is this flag missing on purpose?

with this flag one basically needs just ./configure --with-compat --add-dynamic-module=..