sysutils/rdfind: add support for xxhash

[skozlov]

Summary

In the current form - the support for xxhash depends on whether or not
xxhash is installed in the system where the port is built. See
https://github.com/pauldreik/rdfind#development.

This commit makes (lack of) support for xxhash explicit. If the option
is enabled - the xxhash is added to dependencies and built into rdfind,
if disabled - xxhash support is disabled even if the library is present
in the system.

For the sake of not changing the default behavior - the xxhash support
is disabled by default.

[dtxdf]

Thank you for this.

I don't think there's any problem with enabling XXHASH by default, since it's a lightweight dependency.

[skozlov]

Make XXHASH option default

[skozlov]

In D58398#1339775, @dtxdf wrote:

Thank you for this.

I don't think there's any problem with enabling XXHASH by default, since it's a lightweight dependency.

Done. If you like it - please accept and I'm going to land it :)