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 gentoo: ../../dev/crosstool-NG/ct-ng.trunk/patches/strace/4.5.17/110-dont-use-REG_SYSCALL-for-sh.patch

 -= BEGIN original header =-

 diff -dur strace-4.5.16.orig/process.c strace-4.5.16/process.c

 -= END original header =-

 diff -durN strace-4.5.17.orig/process.c strace-4.5.17/process.c

 --- strace-4.5.17.orig/process.c	2008-07-18 04:16:47.000000000 +0200

 +++ strace-4.5.17/process.c	2008-10-24 21:30:13.000000000 +0200

 @@ -2726,7 +2726,6 @@

         { 4*REG_GBR,            "4*REG_GBR"                             },

         { 4*REG_MACH,           "4*REG_MACH"                            },

         { 4*REG_MACL,           "4*REG_MACL"                            },

 -       { 4*REG_SYSCALL,        "4*REG_SYSCALL"                         },

         { 4*REG_FPUL,           "4*REG_FPUL"                            },

         { 4*REG_FPREG0,         "4*REG_FPREG0"                          },

         { 4*(REG_FPREG0+1),     "4*REG_FPREG1"                          },