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>

 Give preference to target-optimised functions over glibc's ones,

 which in turn ahave precedence over generic ones.

 --- uClibc.orig/libc/Makefile	2005-07-20 08:10:44.000000000 +0200

 +++ uclibc/libc/Makefile	2005-07-28 13:33:40.000000000 +0200

 @@ -59,7 +59,7 @@

  	$(AR) dN 2 $(LIBNAME) $$objs && \

  	$(AR) dN 2 $(LIBNAME) $$objs

  	@for objfile in obj.signal \

 -	                obj.string.generic obj.string.$(TARGET_ARCH) obj.string \

 +	                obj.string obj.string.generic obj.string.$(TARGET_ARCH) \

  	                obj.sysdeps.common obj.sysdeps.$(TARGET_ARCH) ; do \

  		if [ -e $$objfile ] ; then \

  			echo $(AR) $(ARFLAGS) $(LIBNAME) $$objfile ; \