src: use LIBC_SO and LIBNSL_SO from GNU libc, if available

src: use LIBC_SO and LIBNSL_SO from GNU libc, if available

Look for gnu/lib-names.h and use the LIBC_SO and LIBNSL_SO defines to
dlopen libc and libnsl, so the right library is loaded without manually
searching for libc.so.N or libnsl.so.N.

Signed-off-by: Simon Josefsson <simon@josefsson.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>