Initial CMake-based build (#472)

Initial CMake-based build

This knows how to download and build googletest.

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+# Building with CMake
+
+Some portions of this repository are portable beyond iOS and can be built using
+CMake.
+
+## Dependencies
+
+You need:
+  * A C++ compiler
+  * `cmake`
+
+### macOS
+
+You need [Xcode](https://developer.apple.com/xcode/), which you can get from
+the Mac App Store.
+
+You can get other development tools via [homebrew](https://brew.sh). Adjust as
+needed for other package managers.
+```
+brew install cmake
+```
+
+### Ubuntu
+
+Ubuntu Trusty includes CMake 2.8.12 which should be sufficient. Newer versions
+are fine too.
+
+```
+sudo apt-get install cmake
+```
+
+### Windows
+
+An easy way to get development tools is via [Chocolatey](https://chocolatey.org/).
+
+Unfortunately, the `cmake.install` package is semi-broken, so use the portable
+version.
+
+```
+choco install cmake.portable
+```
+
+## Setup
+
+CMake builds out-of source, so create a separate build directory for the target
+you want to work on.
+
+```
+mkdir build
+cd build
+cmake ..
+```
+
+## Testing
+
+Once CMake has run once, you can just run `make` repeatedly and it will
+regenerate Makefiles as needed.
+
+To build everything and run tests:
+```
+make -j all test
+```