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>

 On 20081011, Khem RAJ writes:

  You have to bar gcc trying to build unwinding at stage 1. Try the

  attached patch and see if it helps.

  gcc/config/sh/linux-unwind.h |    3 +++

  1 file changed, 3 insertions(+)

 Index: gcc-4_2-branch/gcc/config/sh/linux-unwind.h

 ===================================================================

 --- gcc-4_2-branch.orig/gcc/config/sh/linux-unwind.h	2008-07-01 17:24:40.000000000 -0700

 +++ gcc-4_2-branch/gcc/config/sh/linux-unwind.h	2008-07-01 17:25:47.000000000 -0700

 @@ -26,6 +26,8 @@ along with this program; see the file CO

  the Free Software Foundation, 51 Franklin Street, Fifth Floor,

  Boston, MA 02110-1301, USA.  */

 +#ifndef inhibit_libc

 +

  /* Do code reading to identify a signal frame, and set the frame

     state data appropriately.  See unwind-dw2.c for the structs.  */

 @@ -251,3 +253,4 @@ sh_fallback_frame_state (struct _Unwind_

    return _URC_NO_REASON;

  }

  #endif /* defined (__SH5__) */

 +#endif /* ifndef inhibit_libc */