CMake Error When Installing

It's not clear to me what Fit's dependencies are, if any, but the following error looks like it expects `boost.hpp` to be present in your repo. This is on the `boost` branch.

```
-- Installing: /opt/install/include/boost/fit/decorate.hpp
-- Installing: /opt/install/include/boost/fit/lazy.hpp
-- Installing: /opt/install/include/boost/fit/version.hpp
-- Installing: /opt/install/include/boost/fit/static.hpp
CMake Error at cmake_install.cmake:40 (file):
file INSTALL cannot find
"/usr/local/src/Fit-a5bb8365ee7b1387ba0ec3a21ac393c7cc962488/include/boost.hpp".
```

I'm running the following script in the root of the repo:

```
#!/bin/bash

mkdir build && cd build \
&& cmake \
-DCMAKE_TOOLCHAIN_FILE=/opt/toolchain.cmake \
-DCMAKE_INSTALL_PREFIX=/opt/install \
-DCMAKE_BUILD_TYPE=Release \
.. \
&& make install
```

cmake version 3.9.0