Use this attribute to make an Objective-C interface method optional.
When applied to a global variable or function, causes it to be emitted to a non-standard object file/executable section.
Use this attribute to attach an Objective-C selector to a method.
Numem attribute hooks for various compilers.
We implement them here to avoid relying on druntime, or phobos.
Some of this code references druntime in dmd, ldc and gcc.