Ignition projects on macOS mix inclusion of system's uuid/uuid.h and linking to homebrew's ossp-uuid

[traversaro]

The homebrew package ossp-uuid provides a library whose functionality is just a subset of the uuid library that is part of the libc/SDK of macOS (see https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man3/uuid.3.html). Exactly to avoid tricky bugs due to this overlap, the ossp-uuid homebrew package installs its headers in ossp\uuid.h (Homebrew/homebrew-core@fdb81d8), while the system uuid library header is installed in uuid\uuid.h . Due to how the UUID::UUID target is created by the ignition-cmake's FindUUID module, the ignition libraries link the library installed by homebrew's ossp-uuid, but apparently they consume the headers of the uuid library as:

#include <uuid/uuid.h>

See for example ignition-transport ( https://github.com/ignitionrobotics/ign-transport/blob/3e84d43aba2c4aeeceda03b3f7ac1647258f6e16/include/ignition/transport/Uuid.hh#L32 ), ignition-common ( https://github.com/ignitionrobotics/ign-common/blob/1c75659001becdb4e8f8f444aaabe540cc32e287/include/ignition/common/Uuid.hh#L31 ) and Gazebo ( https://github.com/osrf/gazebo/blob/6fd426b3949c4ca73fa126cde68f5cc4a59522eb/plugins/rest_web/RestWebPlugin.cc#L27 ).

This means that, despite the apparent logic in CMake, the ignition libraries use the headers from macOS's uuid library, but then link the library provided by ossp-uuid. This is confusing at minimum, but everything works fine because the two libraries are ABI-compatible (at least the symbols used by the ignition libraries). However, in package managers such as conda-forge that do not use the workaround of homebrew of installing libuuid's headers to ossp\uuid.h, this confusion leads to compilation errors in programs that include macOS SDK headers, see for example robotology/robotology-superbuild#513 and conda-forge/libuuid-feedstock#16 .

To solve this confusion, I suggest to modify the FindUUID CMake module to do not search for any library on macOS, but rather just defined UUID::UUID as an empty target, as the necessary symbols are already provided by the macOS SDK. This is similar to what happens for the FindThreads CMake module, that defines the Threads::Threads imported target in any case.