Suppress "missing Optix" cmake warning

[chapulina]

Environment

• OS Version: Ubuntu
• Source or binary build?
  either

Description

• Expected behavior: Our CI should be green, warning-free. But since Fix cmake message types gz-cmake#159 we get warnings for missing dependencies. Since Optix is optional and we don't plan on running CI for it, I think we should disable these warnings.

• Actual behavior: There are CMake warnings

Steps to reproduce

Build ign-rendering without Optix installed.

Output

-- Cannot build component [optix] - Missing: OptiX

[j-rivero]

Quick look at it, if I'm not wrong the logic goes:

• ign_find_package won't find optix so it will call the ign_build_warnings method no matter what other options are.
• ign_configure_build will take the warnings and display them unconditionally as cmake WARNINGS since Fix cmake message types gz-cmake#159

If that is correct, we probably want to add an option to ign_find_package that can indicate that missing support is not a warning.

[chapulina]

option to ign_find_package that can indicate that missing support is not a warning

I'd expect that calling find_package with QUIET wouldn't print a warning, so maybe we can do the same for ign_find_package?

[chapulina]

@j-rivero , I have a suggestion on the PRs above ☝️

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https://github.com/osrf/buildfarmer/issues/181