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 H.J. Lu, as reported there:

http://www.nabble.com/PATCH:-Use-short-for-fnstsw-td14775171.html

fnstsw stores 16bit into %ax. The upper 16bit of %eax is unchanged.

The new assembler (binutils-2.18.50.0.4 and up) will disallow "fnstsw %eax".

diff -dur glibc-2.7.orig/sysdeps/i386/fpu/ftestexcept.c glibc-2.7/sysdeps/i386/fpu/ftestexcept.c

--- glibc-2.7.orig/sysdeps/i386/fpu/ftestexcept.c	2004-03-05 11:14:48.000000000 +0100

+++ glibc-2.7/sysdeps/i386/fpu/ftestexcept.c	2008-06-24 10:54:43.000000000 +0200

@@ -26,7 +26,7 @@

 int

 fetestexcept (int excepts)

 {

-  int temp;

+  short temp;

   int xtemp = 0;

   /* Get current exceptions.  */