configure.ac: use executable hardening where available

Where available, enable stack smashing protection, fortify source,
no-strict-overflow, and read only relocations.

Many Linux distributions automatically enable most of these options.
They are no brainers. The difference introduced here is in asking for a
few more aggressive options. An option to disable the more aggressive
options is provided (--disable-hardening). When set, configure will fall
back to the default CFLAGS on the system - in many cases that will still
be hardened. There is no point in going further than that.

Options enabled are:

-fstack-protector-strong is a relatively new GCC-4.9 feature that is
supposed to give a better balance between performance and protection.
-all is considered too aggressive, but was used in Chromium and other
security critical systems until -strong became available. Follow their
lead and use -strong when possible. clang 6.0 supports -all but not
-strong.

_FORTIFY_SOURCE replaces certain unsafe C str* and mem* functions with
more robust equivalents when the compiler can determine the length of
the buffers involved.

-fno-strict-overflow instructs GCC to not make optimizations based on
the assumption that signed arithmetic will wrap around on overflow (e.g.
(short)0x7FFF + 1 == 0). This prevents the optimizer from doing some
unexpected things. Further improvements should trap signed overflows and
reduce the use of signed to refer to naturally unsigned quantities.

I did not set -fPIE (position independent executables). The critical
function of Open/LibreSSL is as a library, not an executable.

Tested on Ubuntu Linux 14.04.1 LTS, OS X 10.10.1 with "make check".

The code added to m4/ is GPLv3 but con

Signed-off-by: Jim Barlow <jim@purplerock.ca>

scripts/wrap-compiler-for-flag-check

@@ -0,0 +1,25 @@
+#!/bin/sh
+
+# From kmcallister: 
+# https://github.com/kmcallister/autoharden/blob/efaf5a16612589808c276a11536ea9a47071f74b/scripts/wrap-compiler-for-flag-check
+
+# There is no way to make clang's "argument unused" warning fatal.  So when
+# configure checks for supported flags, it runs $CC, $CXX, $LD via this
+# wrapper.
+#
+# Ideally the search string would also include 'clang: ' but this output might
+# depend on clang's argv[0].
+
+if out=`"$@" 2>&1`; then
+  echo "$out"
+  if echo "$out" | grep 'warning: argument unused' >/dev/null; then
+    echo "$0: found clang warning"
+    exit 1
+  else
+    exit 0
+  fi
+else
+  code=$?
+  echo "$out"
+  exit $code
+fi