main: A bunch of miscellaneous improvements

[MacDue]

• Now supports using g++ to compile the generated C++
• Allow setting the compiler to use via the CXX environment variable
• Disable windows deprecation warnings with _CRT_SECURE_NO_DEPRECATE=1 rather than blanket deprecation disable
• Better error messages for when the clang++ or the runtime can't be found
• Add the -p flag to the short usage help

[MacDue]

I've also thrown in a commit to add better error messages for a missing compiler / runtime headers.

Setup Error: Failed to find C++ compiler! Looking for: clang++

or

Setup Error: Failed to find Jakt runtime headers! Looking for: ./runtime

Since the vague IOError messages have confused people a few times now

[AtkinsSJ]

Closed as this is a change for the now-gone Rust-based compiler. Feel free to port your changes to the new compiler, if they're not already added, and open a new PR. :^)