Update build system to allow overriding CXX flags and using clang on Linux

[azeey]

🎉 New feature

Summary

Before this, CMAKE_CXX_FLAGS were unset by the main CMakeLists.txt in https://github.com/ignitionrobotics/sdformat/blob/8fca2f1409fca6e4867178346590b1c628745315/CMakeLists.txt#L164
This makes it impossible to pass additional flags at build time, such as the ones needed for building with ASAN. If there's a particular reason I'm not aware of for unsetting CMAKE_CXX_FLAGS, I leave it to those with more CMake knowledge to tell me.

This PR also updates the logic to add the -stdlib=libc++ flag only on macOS.

Test it

Build with cmake .. -DCMAKE_BUILD_TYPE=Debug -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_C_COMPILER=clang- -DCMAKE_CXX_FLAGS="-fsanitize=address"

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge