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 -urN uClibc-dist/libc/sysdeps/linux/common/bits/uClibc_touplow.h uClibc/libc/sysdeps/linux/common/bits/uClibc_touplow.h

 --- uClibc-dist/libc/sysdeps/linux/common/bits/uClibc_touplow.h   Fri Jun  3 13:30:25 2005

 +++ uClibc/libc/sysdeps/linux/common/bits/uClibc_touplow.h   Fri Jun  3 13:30:39 2005

 @@ -34,11 +34,11 @@

  /* glibc uses the equivalent of - typedef __int32_t __ctype_touplow_t; */

 -typedef __uint16_t __ctype_mask_t;

 +typedef __uint32_t __ctype_mask_t;

  #ifdef __UCLIBC_HAS_CTYPE_SIGNED__

 -typedef __int16_t __ctype_touplow_t;

 +typedef __int32_t __ctype_touplow_t;

  #define __UCLIBC_CTYPE_B_TBL_OFFSET       128

  #define __UCLIBC_CTYPE_TO_TBL_OFFSET      128