(gl_PREREQ_GETADDRINFO): Use AC_GNU_SOURCE,

(gl_PREREQ_GETADDRINFO): Use AC_GNU_SOURCE,
GNU declares only getaddrinfo as an (POSIX) extension.
Check for sys/types.h, sys/socket.h, and netdb.h too,
needed by getaddrinfo.h.
Check if getaddrinfo, freeaddrinfo and gai_strerror are
declared by the POSIX headers.
Check if struct addrinfo is declared.