Remove unix-specific fPIC flag

[mjcarroll]

The exec_program call now raises a cmake warning in sufficiently new cmake versions, it was first triggered here: https://build.osrfoundation.org/job/gz_plugin-ci-pr_any-homebrew-amd64/4/cmake/

With the warning: Policy CMP0153 is not set: The exec_program command should not be called.

The policy is introduced in 3.28, which is probably why we are seeing it on homebrew first: https://cmake.org/cmake/help/latest/policy/CMP0153.html

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Once I started looking at what was happening here, though, I think that we can remove the logic entirely.

Some things I noticed that I think are wrong:

• We are explicitly overriding CMAKE_SYSTEM_PROCESSOR with the contents of uname.
  • In the default case CMAKE_SYSTEM_PROCESSOR should be set to CMAKE_HOST_SYSTEM_PROCESSOR (See https://cmake.org/cmake/help/latest/variable/CMAKE_HOST_SYSTEM_PROCESSOR.html)
  • In the cross compilation case, the toolchain file sets CMAKE_SYSTEM_PROCESSOR
  • That means this is at best duplicate work, but is incorrectly overriding CMAKE_SYSTEM_PROCESSOR if someone is cross-compiling (I don't think we currently support this, but this would make it impossible).
• We are adding fPIC to libraries for x86_64 processors.
  • This should actually be controlled with https://cmake.org/cmake/help/latest/prop_tgt/POSITION_INDEPENDENT_CODE.html
  • POSITION_INDEPENDENT_CODE is automatically added in the case of MODULE or SHARED library types, which would be the same as the logic here.
  • We are also excluding ARM/ARM64 in this logic, but I think it doesn't matter because CMake is already doing the right thing under the hood.

[azeey]

Makes sense to me. Not sure why these were needed in the first place.

[azeey]

cc @claraberendsen