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>

many ports hit this warning:

../sysdeps/unix/sysv/linux/check_native.c:46: warning: implicit declaration of function 'memset'

../sysdeps/unix/sysv/linux/check_native.c:46: warning: implicit declaration of function 'memset'

snipped from suse

diff -durN glibc-2.12.1.orig/sysdeps/unix/sysv/linux/check_native.c glibc-2.12.1/sysdeps/unix/sysv/linux/check_native.c

--- glibc-2.12.1.orig/sysdeps/unix/sysv/linux/check_native.c	2007-11-24 04:12:17.000000000 +0100

+++ glibc-2.12.1/sysdeps/unix/sysv/linux/check_native.c	2009-11-13 00:50:11.000000000 +0100

@@ -23,6 +23,7 @@

 #include <stddef.h>

 #include <stdint.h>

 #include <stdlib.h>

+#include <string.h>

 #include <time.h>

 #include <unistd.h>

 #include <net/if.h>