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 -dur binutils-2.17.old/ld/emultempl/elf32.em binutils-2.17/ld/emultempl/elf32.em

 --- binutils-2.17.old/ld/emultempl/elf32.em	2006-06-12 15:05:04.000000000 +0200

 +++ binutils-2.17/ld/emultempl/elf32.em	2007-05-01 18:26:13.000000000 +0200

 @@ -970,6 +970,8 @@

  	      && command_line.rpath == NULL)

  	    {

  	      lib_path = (const char *) getenv ("LD_RUN_PATH");

 +	      if ((lib_path) && (strlen (lib_path) == 0))

 +		  lib_path = NULL;

  	      if (gld${EMULATION_NAME}_search_needed (lib_path, &n,

  						      force))

  		break;

 @@ -1154,6 +1156,8 @@

    rpath = command_line.rpath;

    if (rpath == NULL)

      rpath = (const char *) getenv ("LD_RUN_PATH");

 +  if ((rpath) && (strlen (rpath) == 0))

 +      rpath = NULL;

    if (! (bfd_elf_size_dynamic_sections

  	 (output_bfd, command_line.soname, rpath,

  	  command_line.filter_shlib,