In a previous change, I forgot to fix the DLLs to
also set the target_compile_features() as needed.

Some amount of refactoring to expose the necessary
variables and functions in AbseilDll.cmake

Fixes #1116

PiperOrigin-RevId: 485101834
Change-Id: I5cd0eff9e20c0ddf48c364f917e40d66df0aac17