Fixed compiler error with older macOS SDKs
Stop AssertMacros.h from defining its macros without underscore prefixes, which pollute the global namespace and cause us build issues. This is default as of the macOS 10.13 SDK, but needed for older SDKs.
Without this, the check
function in vtkSMPTools.h conflicts with the check
macro in AssertMacros.h.
Backport: release