__int64 was the 64bit type for Visual Studio before it added support for "long long" with VS2013. I think this was used to build glib with mingw and make the result usable for VS 6.0 which didn't support "long long" (??)
Given that newer MSVC links against a different crt and mixing is not supported and everything supports "long long" nowadays just remove it.
This is also a cleanup for printf format changes needed for #1497 (closed)