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>

 # Build script for strace

 do_debug_strace_get() {

     CT_GetFile "strace-${CT_STRACE_VERSION}" http://mesh.dl.sourceforge.net/sourceforge/strace/

     # Downloading from sourceforge leaves garbage, cleanup

     CT_DoExecLog ALL rm -f "${CT_TARBALLS_DIR}/showfiles.php"*

 }

 do_debug_strace_extract() {

     CT_Extract "strace-${CT_STRACE_VERSION}"

     CT_Patch "strace" "${CT_STRACE_VERSION}"

 }

 do_debug_strace_build() {

     CT_DoStep INFO "Installing strace"

     mkdir -p "${CT_BUILD_DIR}/build-strace"

     CT_Pushd "${CT_BUILD_DIR}/build-strace"

     CT_DoLog EXTRA "Configuring strace"

     CT_DoExecLog CFG                                        \

     "${CT_SRC_DIR}/strace-${CT_STRACE_VERSION}/configure"   \

         --build=${CT_BUILD}                                 \

         --host=${CT_TARGET}                                 \

         --prefix=/usr

     CT_DoLog EXTRA "Building strace"

     CT_DoExecLog ALL make

     CT_DoLog EXTRA "Installing strace"

     CT_DoExecLog ALL make DESTDIR="${CT_DEBUGROOT_DIR}" install

     CT_Popd

     CT_EndStep

 }