build: Check -lsocket -lnsl for Solaris.

* configure.ac: Check inet_addr for -lnsl, and socket for -lsocket.

Once I used LIB_SOCKET_NSL for the variable name, but it is already
used by AX_LIB_SOCKET_NSL in autoconf-archive as an alias of the
macro. So, I changed the variable name.

• GnuPG-bug-id: T3869
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>