Allow building as Objective-C++

Under C++, the libgit2 enums are their own non-integral type that cannot
be used as an underlying representation in an enum definition themselves.

There are other fixes for this for this (i.e. C++11 `std::underlying_type< gt_enum_t >::type`), but since I'm unsure why the underlying type is even being specified, or if Objective-C++ fixes will be accepted, I don't know how elaborate to get with this.

I'd like to know what case specifying the storage is expected to fix, given that both Objective-Git and libgit2 are going to have to be ABI compatible to even link, I expect the enums mapped with the same count and values as the libgit2 ones to be implicitly identical in storage.