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 m4

 CT_M4_VERSION=1.4.13

 do_companion_tools_m4_get() {

     CT_GetFile "m4-${CT_M4_VERSION}" \

                {ftp,http}://ftp.gnu.org/gnu/m4

 }

 do_companion_tools_m4_extract() {

     CT_Extract "m4-${CT_M4_VERSION}"

     CT_Patch "m4" "${CT_M4_VERSION}"

 }

 do_companion_tools_m4_build() {

     CT_DoStep EXTRA "Installing m4"

     mkdir -p "${CT_BUILD_DIR}/build-m4"

     CT_Pushd "${CT_BUILD_DIR}/build-m4"

     CT_DoExecLog CFG \

     "${CT_SRC_DIR}/m4-${CT_M4_VERSION}/configure" \

         --prefix="${CT_BUILDTOOLS_PREFIX_DIR}"

     CT_DoExecLog ALL make

     CT_DoExecLog ALL make install

     CT_Popd

     CT_EndStep

 }