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>

 # mingw32 config options

 ## depends on EXPERIMENTAL && ( ARCH_x86 && ARCH_32 )

 ##

 ## select MINGW32

 ##

 ## help Build a toolchain targeting systems running Windows as host

 choice

     bool

     prompt "Windows api version"

 # Don't remove next line

 # CT_INSERT_VERSION_BELOW

 config W32API_V_3_14

     bool

     prompt "3.14"

 config W32API_V_select

     bool

     prompt "Other version"

 endchoice

 config W32API_VERSION

     string

     prompt "W32 api version" if W32API_V_select

 # Don't remove next line

 # CT_INSERT_VERSION_STRING_BELOW

     default "3.14" if W32API_V_3_14

     help

       Enter the version number of the windows api files to use