configure.ac: Modify clang check to save result to a variable

...and leave a note that -Qunused-arguments is being applied to CFLAGS
not LDFLAGS, probably in error.
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".

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