By default the cmake files are installed to "<prefix>/lib/cmake/cppformat".
This can now be customized using the `FMT_CMAKE_DIR` cache variable.
Valid paths are documented in the cmake manual [1].
e.g., install the cmake files to "<prefix>/share/cmake/cppformat"
```
cmake -DFMT_CMAKE_DIR=share/cmake/cppformat <path-to-source>
```
Tip: Try `make package` to examine the result.
[1] https://cmake.org/cmake/help/v3.0/command/find_package.html
