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 -u binutils-2.18-orig/configure binutils-2.18/configure

 --- binutils-2.18-orig/configure	2007-08-06 16:29:40.000000000 -0400

 +++ binutils-2.18/configure	2007-09-27 22:41:51.000000000 -0400

 @@ -6125,10 +6125,10 @@

    *" texinfo "*) MAKEINFO='$$r/$(BUILD_SUBDIR)/texinfo/makeinfo/makeinfo' ;;

    *)

 -    # For an installed makeinfo, we require it to be from texinfo 4.4 or

 +    # For an installed makeinfo, we require it to be from texinfo 4.6 or

      # higher, else we use the "missing" dummy.

      if ${MAKEINFO} --version \

 -       | egrep 'texinfo[^0-9]*([1-3][0-9]|4\.[4-9]|[5-9])' >/dev/null 2>&1; then

 +       | egrep 'texinfo[^0-9]*(4\.([6-9]|[1-9][0-9])|[5-9]|[1-9][0-9])' >/dev/null 2>&1; then

        :

      else

        MAKEINFO="$MISSING makeinfo"