Fix autoconfiguration of libraries

Fix autoconfiguration of libraries

configure checked for library functions with LIBS instead of
LIBS_server, which could break detection of getaddrinfo() on systems
where LIB_SOCKET isn't empty.

GNUmakefile put @PTHREAD_LIBS@ only in LDLIBS, which breaks linking of
server and possibly client on systems where it is not empty.

Broken in commit 8b778634.