AGS 4: provide standalone builds of the new compiler

[ericoporto]

The new compiler in ags4 is missing a command line front end. It's necessary to decide one of two approaches.

• Use the existing frontend for the old compiler but use macro/template so it's used in building the new one
• Write it's own frontend for the new compiler, use the same code as baseline but make it it's own thing

It needs it's own CMake script to build it too!

After this, it also needs to be added to the CI - so it can build and be tested in other platforms.

[ivan-mogilko]

The two compiler libraries are using same interface, and similar set of options, so I think it's better to use existing frontend code to build both using either a compile-time or runtime switch.
If there are any incompatible new options then these should be simply ignored by the old compiler.

That is, build 2 programs using same "launcher" code but linking 2 different compiler libs, or build 1 program with both compiler libs linked and switch between them using a cmd arg.

Well, I guess having separate program is better, maybe call it agscc2.

[ericoporto]

I am looking into this

• the preprocessor is using bits of the old compiler, so I need to refactor to it's own thing
  • it's the unused macrotable, so that is quite easy it appears. Done in Compiler: move macrotable to preprocessor sources #2409
  • it also uses some minor helper functions. Done in Compiler: preprocessor can build without compiler #2410 and Compiler: removed duplicates of standard cctype functions #2423
• the new compiler has a very different interface from the old one
  • all flags come into a int64, old compiler had an interface to set each
  • scripts must be concatenated before, old compiler had an interface to feed things
  • the new compiler uses a message handler, the old one you just got the error

I am thinking there's quite a lot of stuff different that I need to pickout to some file and then use a different file depending on compiler...

But it would be super easy to make a new frontend from duplicating the code and later figure what the hell they have in common - which could then go into a compiler_frontend_lib of sorts.