Make hardfork.hpp autogenerated from hardforks.d directory

[theoreticalbts]

There are always a ton of merge conflicts in hardfork.hpp so we should make it auto-generated from a directory of files, each containing one line. Combine these files with find hardforks.d -name "*.hf" | sort | xargs cat > hardfork.hpp but for cross-platform building purposes we need to implement the shell one-liner in a cross-platform way.

[theoreticalbts]

I was thinking we'd have to write a C++ program for the shell one-liner, but it turns out cmake is powerful enough to sort and concatenate a list of files.

[theoreticalbts]

Implemented.