* Still respects "-Dtarget" unity/nonunity selection. Still defaults to
unity.
* Adds a new target (rippled_classic or rippled_unity) depending on
unity/nonunity selection.
* New target does not build by default.
eg. Use `cmake --build . -target rippled_classic`
* Copy all config variables (Debug/Release to DebugClassic/ReleaseClassic) for nonunity builds
* CI uses the more generic "cmake --build" command
* Simplify Travis APT config.
* Automatically retry Travis build and test script. Will result in fewer
false negatives.
* Travis install scripts use absolute paths.
* Build a library of cmake functions for reuse.
* Disallow cmake builds in project root.
* Disallow cmake default 32-bit Visual Studio builds.
* Add several missing nonunity / header files, including all unit tests to
cmake.
* Change gcc.debug.nounity Travis build to use cmake, instead of adding
builds.
* Change Appveyor build to cmake. Eliminates most spurious failures, which
are caused by python or scons failing to download.
