Build system supporting C++14 and C++1z flags for upcoming standards.

This is related to closed issue #21. Given that C++14 is complete and
many features of said standard are already available in popular compilers,
it is time that we started using some of the language features involved
for metaprogramming purposes. The same is extended for the next iteration
of the standard, currenly known as C++17. When CLAUSE_ENABLE_CXX14 is a
defined preprocessor macro, C++14 code becomes available. The behavior
is replicated with CLAUSE_ENABLE_CXX17. The autotools build system
instructions have been updated with --enable-cxx14 and --enable-cxx17
during configuration for such a purpose.

The goal is to make 'clause' more easily upgradable standard-wise while
exploring new kinds of constructs without resorting to emulating them
through laborious source code boilerplate (where applicable) first.
Cross-standard support of newer features also becomes feasable, should
the need arise.

Removed appkit/ since it only contained a pointless readme.txt
Cleaning up appkit details from autotools.