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>

 Original patch from: mpf_set_str.c.4.diff

 -= BEGIN original header =-

 -= END original header =-

 diff -durN gmp-4.2.4.orig/mpf/set_str.c gmp-4.2.4/mpf/set_str.c

 --- gmp-4.2.4.orig/mpf/set_str.c	2008-08-25 16:11:37.000000000 +0200

 +++ gmp-4.2.4/mpf/set_str.c	2009-03-08 18:36:16.000000000 +0100

 @@ -137,7 +137,12 @@

        c = (unsigned char) *++str;

      }

 +  /* Default base to decimal.  */

 +  if (base == 0)

 +    base = 10;

 +

    exp_base = base;

 +

    if (base < 0)

      {

        exp_base = 10;

 @@ -165,10 +170,6 @@

  	return -1;

      }

 -  /* Default base to decimal.  */

 -  if (base == 0)

 -    base = 10;

 -

    /* Locate exponent part of the input.  Look from the right of the string,

       since the exponent is usually a lot shorter than the mantissa.  */

    expptr = NULL;