* 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.