Support a clang-based build pipeline

[stefanscherzinger]

Goal

Check and assure that the cartesian_controllers work with the clang compiler.

Steps

• Switch to clang via the CC and CXX environment variables (only Noetic, keep Melodic with GCC)
• Confirm Cannot create IKSolvers if compiled with clang - fine with gcc #99 in CI
• Reproduce this locally
• Debug what's happening
• apply fixes and recheck that GCC still works (checked by CI Melodic)

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• Just found out that I get the same error with GCC when removing src/IKSolver.cpp from add_library(${PROJECT_NAME} . That's a hint..

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Fix #99

[stefanscherzinger]

Doesn't seem too urgent or relevant. Closing this for now.