API change to X()

This works around the "we can't resize Eigen arrays of anything that
lacks a default constructor" problem - we now require output arrays to
already be properly sized, which is the right thing to do for
efficiency, too.
Add autoconf macro for a dependency's dependency
Minor name resolution bugfix

Now that name lookup is checking std::exp() here, we're passing all
tests with MetaPhysicL enabled again.
Typo fix in test
Bugfixes for VexCL compatibility
New constant_clone() metaprogramming trick

This gets us passing all tests with everything enabled.
Cut-and-paste bug fix in config_summary
Bugfix Eigen constant_clone specialization
Avoid a signed/unsigned comparison warning
Workaround for gcc<4.6 cpp bug

Old g++ preprocessors don't report __cplusplus correctly, so let's
resort to ANTIOCH_HAVE_CXX11 to figure out whether we can use decltype
or not.  This helps to fix a hundred false positive warnings about
signed-unsigned-comparison in assertions when using older gcc.
Enable C++11 by default

Even when we're not using VexCL, C++11 support is nice to use for the
ability to use our decltype-enabled assertions and avoid overzealous
compiler warnings.
Fix unused variable warning
Adding Antioch::if_else

This works in my preliminary tests; next step will be to incorporate
it into the actual code and (cross our fingers) fix the VexCL
operator[] slowdowns.
Fix compilation without boost::proto

Some tragically large forward declarations necessary here.
Bugfix in MetaPhysicL if_else specialization
Add Antioch::min(), new zero_clone()

I should have just added min() at the same time as max() for
completeness.

The new zero_clone() allows type mixing; this is useful when
vectorizing calculations like interval()
Fix if_else(Eigen) specializations

No longer trying to do fancy things, so this version creates more temp
variables than I'd like, but it works.
Cut-n-paste typo fix in unit test
Tolerance tweaking

Some modified GPGPU tests came in barely under tolerance.  Let's bump
up the tolerance by a couple bits and print error levels with more
precision if that ever fails again.
First vectorized if_else usage

This is passing tests for correctness now.  It'll be interesting to
see whether it's done anything for performance.