Two modules for ZStandard compression.  The filter module compresses
responses on the fly for clients that announce zstd in Accept-Encoding,
the way the gzip filter does.  The static module serves a pre-compressed
.zst file from disk when one sits beside the requested file, and never
calls into libzstd at all.

This port only installs the module sources.  The modules themselves are
built by the nginx ports when their ZSTD option is enabled.