configure: Use -fstack-protector-strong when available

configure: Use -fstack-protector-strong when available

Testing whether the compiler supports it is a bit tricky.

The obvious AX_APPEND_COMPILE_FLAGS([-fstack-protector-strong])
doesn't suffice, since some ports of the GNU toolchain reportedly pass
this test, then fail to link.  That's because the compiler accepts the
flag, duly emits references to helper code in libc, but libc doesn't
provide, and linking fails.

Instead, use AX_APPEND_LINK_FLAGS with an input source that makes the
compiler emit the extra stack checking code.  This requires the latest
version from the autoconf-archive, so update m4/ax* to commit e3d948b.
Also update m4/my_append_compile_flags.m4 to keep it in sync with
upstream's ax_append_compile_flags.m4.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>