Obsolete add_definitions (https://cmake.org/cmake/help/latest/command/add_definitions.html) in examples/CMakeLists.txt is not the right way to fix the issue.
Build fails when including zenoh-pico as a package. This happens because the package installation is not correct, not because some settings were forgotten. Necessary settings should come from installed package without bothering the user.

You need to add target_compile_definitions (https://cmake.org/cmake/help/latest/command/target_compile_definitions.html#command:target_compile_definitions) to this file: https://github.com/eclipse-zenoh/zenoh-pico/blob/main/PackageConfig.cmake.in

Look as an example to https://github.com/eclipse-zenoh/zenoh-cpp/blob/main/install/PackageConfig.cmake.in#L23

For now we should merge the fix and make the deeper rework on CMake mentioned by @milyin at a later date, created issue #332 for this purpose.