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 ltrace-0.5.2.orig/configure ltrace-0.5.2/configure

--- ltrace-0.5.2.orig/configure	2009-09-14 18:47:45.000000000 +0200

+++ ltrace-0.5.2/configure	2009-09-14 18:49:18.000000000 +0200

@@ -1829,6 +1829,9 @@

   linux-gnu*) host_os=linux-gnu

 esac

 HOST_OS="$host_os"

+case "${HOST_OS}" in

+  linux-*) HOST_OS=linux-gnu;;

+esac

 ac_ext=c

diff -durN ltrace-0.5.2.orig/configure.ac ltrace-0.5.2/configure.ac

--- ltrace-0.5.2.orig/configure.ac	2009-09-14 18:47:45.000000000 +0200

+++ ltrace-0.5.2/configure.ac	2009-09-14 18:49:18.000000000 +0200

@@ -10,6 +10,9 @@

   linux-gnu*) host_os=linux-gnu

 esac

 HOST_OS="$host_os"

+case "${HOST_OS}" in

+  linux-*) HOST_OS=linux-gnu;;

+esac

 AC_SUBST(HOST_OS)

 HOST_ARCH="$host_cpu"

 AC_SUBST(HOST_ARCH)