I don't understand the point of `INCLUDE_LIB`...  your goal is to have `libgit2.dll` instead of `git2.dll`?  I don't know that we can special case everybody's favorite libgit2 name, so if that's the case, can we just do:

    #ifndef LIBGIT2_FILENAME
    # define LIBGIT2_FILENAME "git2.dll"
    #endif

And you can define it to be whatever you want?