[WIP] New build system.

Rearchitecture of the the build system.  A vast simplification.

Changes
- ./configure can produce an XCode project of the Turi source code.
- deps/ can be compiled and installed out of the regular build process.
  Helpful for the XCode project.
- All packages in src/external/ and src/visualization compile into static
  libraries.
- The rest of the code base compiles into a single shared library.  This
  means that most everything occurs in one single CMakeLists.txt file
  using standard cmake commands.
- All headers are installed into targets/include when `make install` is
  called.  The main library is installed into targets/lib.
- The code definitions that reflect compiler oddities (e.g.
  is std::hash<int128_t> defined) are dumped into a single header file,
  src/turi_common.h.  All source files and headers include this file
  first.  This allows other programs to link against this library /
  headers reliably.
- The python part of TuriCreate now builds through the standard
  setup.py method. Cython / pybind11 extensions are compiled and
  packaged using the standard extensions mechanisms and linking against
  the installed Turi headers.

WIP TODO: the python installation, deployment.

Author: Hoyt Koepke

.gitignore

@@ -11,6 +11,7 @@ syntax: glob
 debug/*
 release/*
 profile/*
+src/turi_common.h
 dist/turicreateapi*
 configure.deps
 config.log