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>

 Index: binutils-2.19.51.0.5/ld/ld.texinfo

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

 diff -durN binutils-2.20.1.orig/ld/ld.texinfo binutils-2.20.1/ld/ld.texinfo

 --- binutils-2.20.1.orig/ld/ld.texinfo	2009-07-06 15:48:51.000000000 +0200

 +++ binutils-2.20.1/ld/ld.texinfo	2010-08-17 19:32:16.000000000 +0200

 @@ -2026,8 +2026,9 @@

  systems may not understand them. If you specify

  @option{--enable-new-dtags}, the dynamic tags will be created as needed.

  If you specify @option{--disable-new-dtags}, no new dynamic tags will be

 -created. By default, the new dynamic tags are not created. Note that

 -those options are only available for ELF systems.

 +created. On Gentoo, by default, the new dynamic tags are created (this

 +differs from upstream behaviour). Note that those options are only

 +available for ELF systems.

  @kindex --hash-size=@var{number}

  @item --hash-size=@var{number}