scripts/libc: do not build add-ons by default

Currently, no --enable-add-ons option is passed to libc configure when
"$(do_libc_add_ons_list ,)" is empty, which makes configure automatically search
for present add-ons. In that case, all present add-ons are built, although
no add-on was selected by the user in the config. Moreover, this can make the
configure fail if some non-standard add-ons like eglibc-localedef are present.

This behavior also leads to an inconsistency from a user point of view between
the following cases:
- LIBC_ADDONS_LIST="", LIBC_GLIBC_USE_PORTS=n and THREADS="none" in the config,
which makes "$(do_libc_add_ons_list ,)" return "", so all present add-ons
are built.
- LIBC_ADDONS_LIST="", LIBC_GLIBC_USE_PORTS=n and THREADS!="none" in the
config, which makes "$(do_libc_add_ons_list ,)" return the add-on supporting
the chosen threading implementation, e.g. "nptl", so only this add-on is
built.

This patch disables the building of all add-ons in that case.

It is still possible to build all present add-ons by adding --enable-add-ons to
LIBC_GLIBC_EXTRA_CONFIG_ARRAY.

Signed-off-by: "Benoît THÉBAUDEAU" <benoit.thebaudeau@advansee.com>

 only fork one to assist in stop-start-daemon assumptions about daemon behavior

 http://bugs.gentoo.org/190785

 diff -durN glibc-2.12.1.orig/nscd/nscd.c glibc-2.12.1/nscd/nscd.c

 --- glibc-2.12.1.orig/nscd/nscd.c	2009-02-06 21:10:27.000000000 +0100

 +++ glibc-2.12.1/nscd/nscd.c	2009-11-13 00:50:24.000000000 +0100

 @@ -182,6 +182,9 @@

        if (pid != 0)

  	exit (0);

 +       if (write_pid (_PATH_NSCDPID) < 0)

 +	dbg_log ("%s: %s", _PATH_NSCDPID, strerror (errno));

 +

        int nullfd = open (_PATH_DEVNULL, O_RDWR);

        if (nullfd != -1)

  	{

 @@ -231,12 +234,6 @@

  	for (i = min_close_fd; i < getdtablesize (); i++)

  	  close (i);

 -      pid = fork ();

 -      if (pid == -1)

 -	error (EXIT_FAILURE, errno, _("cannot fork"));

 -      if (pid != 0)

 -	exit (0);

 -

        setsid ();

        if (chdir ("/") != 0)

 @@ -245,9 +242,6 @@

        openlog ("nscd", LOG_CONS | LOG_ODELAY, LOG_DAEMON);

 -      if (write_pid (_PATH_NSCDPID) < 0)

 -	dbg_log ("%s: %s", _PATH_NSCDPID, strerror (errno));

 -

        if (!init_logfile ())

  	dbg_log (_("Could not create log file"));