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>

 diff -durN gcc-3.4.6.orig/gcc/config/sh/sh.h gcc-3.4.6/gcc/config/sh/sh.h

 --- gcc-3.4.6.orig/gcc/config/sh/sh.h	2006-01-12 02:20:32.000000000 +0100

 +++ gcc-3.4.6/gcc/config/sh/sh.h	2007-08-15 23:03:09.000000000 +0200

 @@ -422,6 +422,10 @@

  do {									\

    if (LEVEL)								\

      flag_omit_frame_pointer = -1;					\

 +  if (LEVEL <= 2)							\

 +    {									\

 +      flag_reorder_blocks = 0;						\

 +    }									\

    if (SIZE)								\

      target_flags |= SPACE_BIT;						\

    if (TARGET_SHMEDIA && LEVEL > 1)					\