Config.in: enable FORTIFY_SOURCE, PIC/PIE, RELRO, SSP by default

Enhance security by enabling FORTIFY_SOURCE, PIC/PIE, RELRO and SSP by
default.

For SSP, SSP-all can have a significant impact on performance, so we do
not want to enable that unconditionally; instead we use SSP-strong if
available (since gcc-4.9), and resort to SSP-regular otherwise. People
who really, like really-really want to use SSP-all will still have to
enable it explicitly.

For FORTIFY, level 2 may change the behaviour of some glibc functions,
so may crash conforming programs, so may have adverse effects. As such,
we choose level 1 as the default, as it does not change the behaviour
of any function.

This could help making IoT more secure and fight against the assumption
that buildroot does not support binary hardening (see
https://cyber-itl.org/2019/08/26/iot-data-writeup.html

)

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[yann.morin.1998@free.fr:
  - relax SSP to strong when available, regular otherwise
  - extend commit log to explain why SSP-all is not used
  - extend commit log to explain why FORTIFY level 2 is not used
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>