libc/uclibc: add uClibc 0.9.31

Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>
[Yann E. MORIN: mark it EXPERIMENTAL]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

cc/gcc: fix thread-less final gcc build

If threads are disabled in libc, we don't want to enable them in the
final compiler. Doing so pass the configure stage, but fails latter on
a missing <pthread.h>.

Moreover, we don't want to build libgomp if threads are disabled; its
configure script would fails anyway.

Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>

cc/gcc: fix recent gcc build

This fix missed conversion of CT_GCC_USE_* to CT_CC_GCC_USE_*.

Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>

complibs/gmp: fix patch against 5.0.1

config: allow libc selection in backend mode

In backend mode, allow the upper-layer build system to force the
C library selection.

config: do not force setting kernel and arch

In backend mode, only enforce the arch and/or kernel selection if the
upper-layer build system effectively forces the selection.

scripts: misc help and auto-complete fixes

- don't list samples in the main help screen
- improve the samples listing in list-samples
- don't document the 'config' action, it's long dead
- document the 'V' environment variable
- improve on START, STOP and PREFIX environment variables
- add PREFIX and V to autocomplete
- advertise auto-complete at install time