Create a simple makefile and allow regressions to use it.

[threeifbyair]

One of the frustrating things about cppfront is that I always have to remember the precise incantation to build cppfront. In addition, the regression tests always build cppfront whether or not it needs to be built.

This PR adds a simple Makefile. Nothing special and explicitly marked as optional. In addition, it allows the regression tests to be run with a pre-built version of cppfront. This allows us (for instance) to build a sanitized cppfront and run it through the regressions to see if we get any memory or undefined behavior errors.

[hsutter]

Thanks! @jarzec you're the expert on that, do you think this will improve regression tests, and do you approve merging this PR?

[jarzec]

Thanks! @jarzec you're the expert on that, do you think this will improve regression tests, and do you approve merging this PR?

Some context
A while back, after your CppCon 22 presentation, Cpp2 caught interest of many people (myself included). This interest resulted in a number of PRs with CMake setup for cppfront. At that stage you closed those as you didn't want to add extra dependencies. That is why I proposed a pure bash sctipt that can be used on MasOS, Linux and Windows (through git bash) without any extra deps installed. This is what I used for the GitHub actions running for the regression tests.
The idea of using a build system for cppfront was not buried, though. In the Related Projects section of the wiki you point to a number of forks that propose a CMake/Mason setup.

My thoughts
It turns out GNU Make is installed on the Windows GitHub runners (which I didn't know), so it seems to be a possible way to go.
However, if adding Make for regression tests starts to be an option I would suggest a discussion about a more complete and flexible system like CMake or the like. The source code of cppfront is constantly growing while still being basically a single translation unit with really huge files.
CMake would also greatly facilitate integration of cppfront in other projects.
As I mentioned before, this is might be a good topic for a discussion.

On the other hand, adding a command line option to provide the executable to the run-test.sh wouldn't hurt It would e.g. help with developing new tests without having to recompile cppfront every time the test is run.

@hsutter What do you say?