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>

 timezone data has been split into the package sys-libs/timezone-data

 diff -durN glibc-2.12.1.orig/Makeconfig glibc-2.12.1/Makeconfig

 --- glibc-2.12.1.orig/Makeconfig	2009-11-13 00:50:13.000000000 +0100

 +++ glibc-2.12.1/Makeconfig	2009-11-13 00:50:41.000000000 +0100

 @@ -944,7 +944,7 @@

  	      stdlib stdio-common libio malloc string wcsmbs time dirent    \

  	      grp pwd posix io termios resource misc socket sysvipc gmon    \

  	      gnulib iconv iconvdata wctype manual shadow gshadow po argp   \

 -	      crypt nss localedata timezone rt conform debug		    \

 +	      crypt nss localedata rt conform debug		    \

  	      $(add-on-subdirs) $(dlfcn) $(binfmt-subdir)

  ifndef avoid-generated