cmake: install interface library target if cmake version >= 3.0

[tamaskenez]

Thanks for your implementation of the std::optional!

I added an interface library target to the CMakeLists.txt, many of us use CMake libraries (even header-only ones) this way only. Feel free to suggest changes or reject.

Usage:

In a client project's CMakeLists.txt:

 find_package(akrzemi1_optional REQUIRED)
 ...
 target_link_libraries(mytarget ... akrzemi1::optional ...)

In a client C++ source file:

#include "akrzemi1/optional.hpp"