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>

--- glibc-2.12.1/sysdeps/unix/sysv/linux/kernel-features.h	2010-07-27 14:34:39.000000000 +0300

+++ glibc-2.12.1/sysdeps/unix/sysv/linux/kernel-features.h.new	2011-03-10 18:54:37.686795979 +0200

@@ -437,7 +437,8 @@

    the code.  On PPC they were introduced in 2.6.17-rc1,

    on SH in 2.6.19-rc1.  */

 #if __LINUX_KERNEL_VERSION >= 0x020611 \

-    && (!defined __sh__ || __LINUX_KERNEL_VERSION >= 0x020613)

+    && (!defined __sh__ || __LINUX_KERNEL_VERSION >= 0x020613) \

+    && (!defined __alpha__)

 # define __ASSUME_ATFCTS	1

 #endif