Support using an existing fast_float CMake target

[ajtribick]

Suppose I have both fast_float and scnlib as subdirectories/submodules of my project. If possible, I would like to be able to have scnlib use the existing FastFloat::fast_float target generated by including the fast_float subdirectory rather than trying to find an installed version via find_package or using FetchContent.

This would be useful in situations where the build infrastructure does not support doing downloads at build time.

[eliaskosunen]

In 74c39b7, find_package is only called if the target is not already defined. How's that?

[ajtribick]

Great, thanks a lot!