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-4.2.1.orig/libstdc++-v3/include/ext/rope gcc-4.2.1/libstdc++-v3/include/ext/rope

 --- gcc-4.2.1.orig/libstdc++-v3/include/ext/rope	2006-10-17 13:56:21.000000000 +0200

 +++ gcc-4.2.1/libstdc++-v3/include/ext/rope	2007-08-03 20:34:23.000000000 +0200

 @@ -58,6 +58,9 @@

  #include <bits/allocator.h>

  #include <ext/hash_fun.h>

 +/* cope w/ index defined as macro, SuSv3 proposal */

 +#undef index

 +

  # ifdef __GC

  #   define __GC_CONST const

  # else

 diff -durN gcc-4.2.1.orig/libstdc++-v3/include/ext/ropeimpl.h gcc-4.2.1/libstdc++-v3/include/ext/ropeimpl.h

 --- gcc-4.2.1.orig/libstdc++-v3/include/ext/ropeimpl.h	2006-10-17 13:56:21.000000000 +0200

 +++ gcc-4.2.1/libstdc++-v3/include/ext/ropeimpl.h	2007-08-03 20:34:23.000000000 +0200

 @@ -54,6 +54,9 @@

  #include <ext/memory> // For uninitialized_copy_n

  #include <ext/numeric> // For power

 +/* cope w/ index defined as macro, SuSv3 proposal */

 +#undef index

 +

  _GLIBCXX_BEGIN_NAMESPACE(__gnu_cxx)

    using std::size_t;