If you intend for your library to be used in both C and C++ then your doing yourself a favor by using C/C++ for the simple fact that this is marketing to C++ programmers that they can use the library who may not be aware that that is the case.

so at the bare minimum C/C++ will possibly increase your user base.

(edit)

You should not assume that C++ programmers are going to wade through your library code and rewrite your library with the proper signatures. You as the library writer should have done that and tested it, then advertised to prospective users that your library is ready for use with C++.