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 -durN gcc-3.4.6.orig/gcc/flow.c gcc-3.4.6/gcc/flow.c

 --- gcc-3.4.6.orig/gcc/flow.c	2005-09-01 22:51:09.000000000 +0200

 +++ gcc-3.4.6/gcc/flow.c	2007-08-15 23:00:30.000000000 +0200

 @@ -1884,6 +1884,7 @@

  	  rtx set_src = SET_SRC (pc_set (BB_END (bb)));

  	  rtx cond_true = XEXP (set_src, 0);

  	  rtx reg = XEXP (cond_true, 0);

 + 	  enum rtx_code inv_cond;

  	  if (GET_CODE (reg) == SUBREG)

  	    reg = SUBREG_REG (reg);

 @@ -1892,11 +1893,13 @@

  	     in the form of a comparison of a register against zero.  

  	     If the condition is more complex than that, then it is safe

  	     not to record any information.  */

 -	  if (GET_CODE (reg) == REG

 + 	  inv_cond = reversed_comparison_code (cond_true, BB_END (bb));

 + 	  if (inv_cond != UNKNOWN

 +	      && GET_CODE (reg) == REG

  	      && XEXP (cond_true, 1) == const0_rtx)

  	    {

  	      rtx cond_false

 -		= gen_rtx_fmt_ee (reverse_condition (GET_CODE (cond_true)),

 +		= gen_rtx_fmt_ee (inv_cond,

  				  GET_MODE (cond_true), XEXP (cond_true, 0),

  				  XEXP (cond_true, 1));

  	      if (GET_CODE (XEXP (set_src, 1)) == PC)