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/gcc/Makefile.in gcc-4.2.1/gcc/Makefile.in

 --- gcc-4.2.1.orig/gcc/Makefile.in	2007-05-31 17:37:38.000000000 +0200

 +++ gcc-4.2.1/gcc/Makefile.in	2007-08-03 20:36:14.000000000 +0200

 @@ -2660,7 +2660,7 @@

  # FIXME: writing proper dependencies for this is a *LOT* of work.

  libbackend.o : $(OBJS-common:.o=.c) $(out_file) \

    insn-config.h insn-flags.h insn-codes.h insn-constants.h \

 -  insn-attr.h  $(DATESTAMP) $(BASEVER) $(DEVPHASE)

 +  insn-attr.h  $(DATESTAMP) $(BASEVER) $(DEVPHASE) gcov-iov.h

  	$(CC) $(ALL_CFLAGS) $(ALL_CPPFLAGS) \

  	  -DTARGET_NAME=\"$(target_noncanonical)\" \

  	  -DLOCALEDIR=\"$(localedir)\" \