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>

 http://sources.redhat.com/ml/binutils/2004-04/msg00665.html

 fixes building on some architectures (like m68k/arm/cris/etc...) because

 it does the right thing

 diff -durN glibc-2.12.1.orig/include/libc-symbols.h glibc-2.12.1/include/libc-symbols.h

 --- glibc-2.12.1.orig/include/libc-symbols.h	2009-03-14 00:51:46.000000000 +0100

 +++ glibc-2.12.1/include/libc-symbols.h	2009-11-13 00:50:07.000000000 +0100

 @@ -239,12 +239,12 @@

  #  define __make_section_unallocated(section_string)

  # endif

 -/* Tacking on "\n\t#" to the section name makes gcc put it's bogus

 +/* Tacking on "\n#APP\n\t#" to the section name makes gcc put it's bogus

     section attributes on what looks like a comment to the assembler.  */

  # ifdef HAVE_SECTION_QUOTES

 -#  define __sec_comment "\"\n\t#\""

 +#  define __sec_comment "\"\n#APP\n\t#\""

  # else

 -#  define __sec_comment "\n\t#"

 +#  define __sec_comment "\n#APP\n\t#"

  # endif

  # define link_warning(symbol, msg) \

    __make_section_unallocated (".gnu.warning." #symbol) \