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>

 # This file declares functions to install the kernel headers for mingw

 # Copyright 2009 Bart vdr. Meulen

 # Licensed under the GPL v2. See COPYING in the root of this package

 CT_DoKernelTupleValues() {

     CT_TARGET_KERNEL="mingw32"

     CT_TARGET_SYS=

 }

 do_kernel_get() {

     CT_GetFile "w32api-${CT_W32API_VERSION}-mingw32-src" \

         http://downloads.sourceforge.net/sourceforge/mingw

 }

 do_kernel_extract() {

     CT_Extract "w32api-${CT_W32API_VERSION}-mingw32-src"

 }

 do_kernel_headers() {

     CT_DoStep INFO "Installing kernel headers"

     mkdir -p "${CT_HEADERS_DIR}"

     cp -r ${CT_SRC_DIR}/w32api-${CT_W32API_VERSION}-mingw32/include/* \

           ${CT_HEADERS_DIR}

     CT_EndStep

 }