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>

 Fix for psim build failure due to -lz

 See http://sourceware.org/bugzilla/show_bug.cgi?id=12202

 --- gdb-7.2.orig/sim/ppc/Makefile.in

 +++ gdb-7.2/sim/ppc/Makefile.in

 @@ -551,7 +551,7 @@

  PACKAGE_OBJ = @sim_pk_obj@

 -psim: $(TARGETLIB) main.o $(LIBIBERTY_LIB) $(BFD_LIB) $(LIBS) $(LIBINTL_DEP)

 +psim: $(TARGETLIB) main.o $(LIBIBERTY_LIB) $(BFD_LIB) $(LIBINTL_DEP)

  	$(CC) $(CFLAGS) $(SIM_CFLAGS) $(LDFLAGS) -o psim$(EXEEXT) main.o $(TARGETLIB) $(BFD_LIB) $(LIBINTL) $(LIBIBERTY_LIB) $(LIBS)

  run: psim