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>

     1.1 --- a/scripts/build/libc/glibc-eglibc.sh-common	Thu Jul 28 21:55:20 2011 +0200
     1.2 +++ b/scripts/build/libc/glibc-eglibc.sh-common	Thu Jul 28 22:09:31 2011 +0200
     1.3 @@ -142,7 +142,7 @@
     1.4      fi
     1.5  
     1.6      case "$(do_libc_add_ons_list ,)" in
     1.7 -        "") ;;
     1.8 +        "") extra_config+=("--enable-add-ons=no");;
     1.9          *)  extra_config+=("--enable-add-ons=$(do_libc_add_ons_list ,)");;
    1.10      esac
    1.11