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>

 # Wrapper to build the debug facilities

 # List all debug facilities, and parse their scripts

 CT_DEBUG_FACILITY_LIST=

 for f in "${CT_LIB_DIR}/scripts/build/debug/"*.sh; do

     _f="$(basename "${f}" .sh)"

     _f="${_f#???-}"

     __f="CT_DEBUG_${_f}"

     if [ "${!__f}" = "y" ]; then

         CT_DoLog DEBUG "Enabling debug '${_f}'"

         . "${f}"

         CT_DEBUG_FACILITY_LIST="${CT_DEBUG_FACILITY_LIST} ${_f}"

     else

         CT_DoLog DEBUG "Disabling debug '${_f}'"

     fi

 done

 # Download the debug facilities

 do_debug_get() {

     for f in ${CT_DEBUG_FACILITY_LIST}; do

         do_debug_${f}_get

     done

 }

 # Extract and patch the debug facilities

 do_debug_extract() {

     for f in ${CT_DEBUG_FACILITY_LIST}; do

         do_debug_${f}_extract

     done

 }

 # Build the debug facilities

 do_debug() {

     for f in ${CT_DEBUG_FACILITY_LIST}; do

         do_debug_${f}_build

     done

 }