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>
When AC_PROG_CC detects GCC, Make.mk adds a bunch of flags to CFLAGS.
Works only for flags that any version of gcc in use accepts.
Instead, make configure add the flags that actually work to CFLAGS.
This will let us add flags that work only for some compilers.
The new autoconf macros are from autoconf-archive v2015.02.24.
Unfortunately, AX_APPEND_COMPILE_FLAGS doesn't work reliably for
-Wno-*: gcc complains about unknown -Wno-foo only when other
diagnostics are being produced. Test -Wfoo instead of -Wno-foo, and
rename to MY_APPEND_COMPILE_FLAGS.
Signed-off-by: Markus Armbruster <armbru@pond.sub.org>
