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>

 # ltrace

 ## depends on ! BACKEND

 ##

 ## select LIBELF_TARGET

 ##

 ## help ltrace is a program that simply runs the specified command until it exits.

 ## help It intercepts and records the dynamic library calls which are called by

 ## help the executed process and the signals which are received by that process.

 ## help It can also intercept and print the system calls executed by the program.

 choice

     bool

     prompt "ltrace version"

 # Don't remove next line

 # CT_INSERT_VERSION_BELOW

 config LTRACE_V_0_5_3

     bool

     prompt "0.5.3"

     select LTRACE_0_5_3_or_later

 config LTRACE_V_0_5_2

     bool

     prompt "0.5.2"

 config LTRACE_V_0_5

     bool

     prompt "0.5 (OBSOLETE)"

     depends on OBSOLETE

 config LTRACE_V_0_4

     bool

     prompt "0.4 (OBSOLETE)"

     depends on OBSOLETE

 endchoice

 config LTRACE_VERSION

     string

 # Don't remove next line

 # CT_INSERT_VERSION_STRING_BELOW

     default "0.5.3" if LTRACE_V_0_5_3

     default "0.5.2" if LTRACE_V_0_5_2

     default "0.5.1" if LTRACE_V_0_5_1

     default "0.5" if LTRACE_V_0_5

     default "0.4" if LTRACE_V_0_4

 config LTRACE_0_5_3_or_later

     bool