[WIP] New build system.

[hoytak]

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, at configure time. All source files and headers now 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 building.

To review: Essentially it's all in the root CMakeLists.txt file, with some changes to ./configure. Just focus on those two.

[guihao-liang]

forked in #3065.