Compilation improvements

I applied these two changes in order to build libsuinput successfully on Fedora 20, and link it with a C++ project. I think the change to suinput.h is straightforward, and just ensures the right calling conventions are used if the header is compiled as C++. The change to Makefile.am I'm much less certain of - but clearly the .0 extension is not available on all environments, and I believe that the regular .so file should be a symbolic link to the relevant version anyway. Feel free to correct me if I'm wrong.

I hope these are useful!