Since Zeek version 5.0, support for Spicy is built right into Zeek by default. To confirm that Spicy is indeed available, you can inspect the output of zeek -N:

# zeek -N Zeek::Spicy
Zeek::Spicy - Support for Spicy parsers (*.hlto) (built-in)

It remains possible to build Zeek against an external Spicy installation through Zeek’s configure option --with-spicy=PATH, where PATH points to the Spicy installation directory. In that case, you also need to ensure that the Spicy tools (e.g., spicyc, spicy-config) are available in PATH.