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>

 ripped from Debian

  sysdeps/unix/sysv/linux/posix_fadvise.c |   13 +++++++++++++

  1 file changed, 13 insertions(+)

 diff -durN glibc-2.12.1.orig/sysdeps/unix/sysv/linux/posix_fadvise.c glibc-2.12.1/sysdeps/unix/sysv/linux/posix_fadvise.c

 --- glibc-2.12.1.orig/sysdeps/unix/sysv/linux/posix_fadvise.c	2003-08-17 02:36:22.000000000 +0200

 +++ glibc-2.12.1/sysdeps/unix/sysv/linux/posix_fadvise.c	2009-11-13 00:50:03.000000000 +0100

 @@ -35,6 +35,19 @@

      return INTERNAL_SYSCALL_ERRNO (ret, err);

    return 0;

  #else

 +# ifdef __NR_fadvise64_64

 +  INTERNAL_SYSCALL_DECL (err);  

 +  int ret = INTERNAL_SYSCALL (fadvise64_64, err, 6, fd,

 +			      __LONG_LONG_PAIR ((long) (offset >> 31),

 +						(long) offset),

 +			      __LONG_LONG_PAIR ((long) (len >> 31),

 +						(long) len),

 +			      advise);

 +  if (INTERNAL_SYSCALL_ERROR_P (ret, err))

 +    return INTERNAL_SYSCALL_ERRNO (ret, err);

 +  return 0;

 +# else

    return ENOSYS;

 +# endif

  #endif

  }