Reference: https://stackoverflow.com/a/23766303
Documentation: https://cmake.org/cmake/help/latest/command/install.html#installing-directories

This is much simpler than using the C functions. As a bonus, I/O errors
throw exceptions.

TNL is not supposed to have any external dependencies, so an empty
config file is useless. If some marginal parts of the project (such as
code for image processing) need to depend on some non-standard library,
they should be split into a separate module/project so that the
dependencies can be properly specified.

For now, the macros are specified as compiler parameters. Client
software using the affected headers will have to do the same.

This allows int-vectors to be multiplied by float-scalars, producing
accurate int values (e.g. 4.2 * 10 == 42).