fix USD -> SDF converter build

[adlarkin]

Signed-off-by: Ashton Larkin 42042756+adlarkin@users.noreply.github.com

🦟 Bug fix

requires gazebosim/gz-math#366

Summary

This builds on top of #819 to resolve build issues I faced when trying to setup the USD -> SDF converter. The main changes are:

1. Remove usage of the c++ filesystem library and use ignition::common::FileSystem instead
2. Enforce CGAL version 5 in the CMakeLists.txt
3. Add symbol visibility to resolve linking errors
4. Remove things like unused variables to help with compiler warnings

Note: CGAL version 5 binaries are not available on bionic. To setup CGAL 5 on bionic, follow steps 3.2 and 4 here: https://doc.cgal.org/latest/Manual/usage.html#secusingwebsite

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[adlarkin]

@ahcorde @koonpeng: in order to keep the commit history clean in #819, we should merge #819 into the USD -> SDF converter first before merging this PR. Also, I faced a build error from ign-math, which I believe is resolved by gazebosim/gz-math#366. So, if it turns out that we do need this fix upstream in ign-math, then we need to make a release of ign-math before merging this as well.

[koonpeng]

@ahcorde @koonpeng: in order to keep the commit history clean in #819, we should merge #819 into the USD -> SDF converter first before merging this PR. Also, I faced a build error from ign-math, which I believe is resolved by gazebosim/gz-math#366. So, if it turns out that we do need this fix upstream in ign-math, then we need to make a release of ign-math before merging this as well.

Should this target ahcorde/usd/prototype_main instead? I should remove CGAL in #819 since it should no longer be used.

[ahcorde]

I cherry-picked this commit in the ahcorde/usd/prototype_main branch