configure.ac: add --disable-hardening option

Allow the user to disable hardening which is enabled by default since
version 02092020 and
https://github.com/rurban/safeclib/commit/caa4408eb4a5c767f0474258af3b6ccf444e10aa
to avoid the following build failure when the toolchain doesn't support
stack-protector:

/home/buildroot/autobuild/instance-3/output-1/host/opt/ext-toolchain/m68k-buildroot-uclinux-uclibc/bin/ld.real: ../src/.libs/libsafec-3.6.0.a(safe_mem_constraint.o): in function `handle_mem_bos_chk_warn':
safe_mem_constraint.c:(.text+0x40): undefined reference to `__stack_chk_guard'

Fixes:
 - http://autobuild.buildroot.org/results/a481ee2d26a094358b0298617cce691be3077f22



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>