netdb: Make it work in C++ mode. * lib/netdb.in.h (struct addrinfo): In C++, define as a C struct. (getaddrinfo, freeaddrinfo, getnameinfo): Use macros from c++defs module. * m4/netdb_h.m4 (gl_NETDB_MODULE_INDICATOR): Invoke gl_MODULE_INDICATOR_FOR_TESTS. * modules/netdb-tests (Depends-on): Add netdb-c++-tests. * modules/netdb-c++-tests: New file. * tests/test-netdb-c++.cc: New file.