Revert "Patch by Dan Liew: "Improved AutoRegen script so that it supports newer versions of autoconf""

The entire point of this version check in this script is so that configure is
only generated with one exact version of autotools, so that it doesn't get
spurious diffs from people generating it with different versions. This is the
approach used by LLVM, and I'd like to stick to it. Most people don't regenerate
these files, so it falls on the people who do to install the appropriate
versions of the tools.

If we are going to change this policy, we should just drop the version checks completely.
Regenerate configure with sanctioned autoconf version.
Do not require <sys/capability.h>, which is Linux specific.
Do not require mallinfo(), which is Linux specific.
Add support for getting memory usage on Darwin.
Tweak the workarounds for multiple definition of PACKAGE_* macros from config.h files.

 - There seems to be a better solution for this by defining a macro prefix, per:

     http://www.gnu.org/software/autoconf-archive/ax_prefix_config_h.html

   but I have no experience with that and it looks like it might involve
   rewriting a bunch of our macro checks.
Regenerate configure.
When building against libc++ (vs libstdcxx), use standard unordered_{map,set} includes.

 - I'm not sure what the status of libstdcxx's c++11 support is. It may be we
   can just move over to <unordered_map> everywhere, but I don't have a Linux
   test machine handy at the moment.
XFAIL LargeReturnTypes.cpp on Darwin, it depends indirectly on using libstdcxx.
Update FloatingPointOps concrete test case for LLVM changes.
Update testingUtils to use printf(), putchar() isn't a known external function anymore.
Update Concrete test Makefile for LLVM changes.
Rewrite ConcreteTest.py to only run one test at a time.
Fix up Concrete Makefile to not remove outputs, so tests can run in parallel.
Add support for testing Concrete tests via lit.

Fixes #96.
Fix up ConstantExpr to be deterministic with 64-bit addresses.
Add a README for the Concrete tests.
[tests] Update ConcreteTest to use --output-dir.
[tests] Set --output-dir on all test runs, in support of running tests in parallel.

 - It would be nice if there was an easier way to do this that didn't involve
   editing all of the tests (like running each test in its own directory), but
   this approach fixes #146 and doesn't involve changing 'lit' or writing a
   wrapper harness. My assumption is a lot of tests start are derived from
   another one, so hopefully following this convention won't be burdensome, and
   I updated 'make check' so that it will produce an error if any test runs klee
   without --output-dir (by checking for the existing of klee-last files).

 - This also helps with #147 but I still can't fully run tests in parallel (I
   start hitting STP errors).
[tests] Dramatically scale back the forks limit for our ImmutableSet test.

 - This one test was taking about half the test time, and it definitely isn't
   worth that much value as a test. We probably should have some kind of long
   running tests infrastructure for things where we actually want to test KLEE
   on real programs.