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>

 http://sourceware.org/ml/libc-alpha/2002-10/msg00392.html

 diff -durN glibc-2.12.1.orig/glibc-ports-2.12.1/sysdeps/mips/fpu_control.h glibc-2.12.1/glibc-ports-2.12.1/sysdeps/mips/fpu_control.h

 --- glibc-2.12.1.orig/glibc-ports-2.12.1/sysdeps/mips/fpu_control.h	2009-05-16 10:36:20.000000000 +0200

 +++ glibc-2.12.1/glibc-ports-2.12.1/sysdeps/mips/fpu_control.h	2009-11-13 00:51:31.000000000 +0100

 @@ -86,7 +86,7 @@

  #define _FPU_RC_UP      0x2

  #define _FPU_RC_DOWN    0x3

 -#define _FPU_RESERVED 0xfe3c0000  /* Reserved bits in cw */

 +#define _FPU_RESERVED 0xfebc0000  /* Reserved bits in cw */

  /* The fdlibm code requires strict IEEE double precision arithmetic,

 diff -durN glibc-2.12.1.orig/ports/sysdeps/mips/fpu_control.h glibc-2.12.1/ports/sysdeps/mips/fpu_control.h