FreeBSD 12 includes clang version 6.0.1 (imported a few days ago, I tested with 6.0.0).
This compiler is really picky about standards and a few lines required changing:
in AtlasObjectImpl.ccp clang was erroring out due to the calls to add and set being considered ambiguous.
I took a brutal approach and duplicated the code inside the functions calling them directly. This can be made better, but I'm not a C++ expert, so I did not spend too much time thinking about it.
In MapDialog.cpp and ScenarioEditor.cpp the calls to wxString::wc_str() return wchar_t*, but the constructor they are passed into expects std::wstring so I changed the call to wxString::ToStdWstring(), returning the expected type.