Provide flag for enabling Link-Time Optimization

[toxtran]

Hi!

I noticed that in the CMakeLists.txt file Interprocedural Optimization (IPO, or Link-Lime Optimization (LTO)) is not presented for the project. I suggest to provide an option to enable it on since it will reduce the binary size (always a good thing to have) and will likely improve the application's performance a bit. If you want to read a bit more about LTO, I can recommend starting from from this (GCC) and this (LLVM). CMake can determine LTO support with the check_ipo_supported.

In the future, users will be able to choose do they want to use LTO (and get benefits from it) or not.

Thank you!

[seekstar]

Thank you for letting me know about this technique. But in my understanding -flto is used when compiling a .cpp file, while rusty-cpp is currently a header-only library. Doesn't enabling it in the project using rusty-cpp make more sense?