Create a standalone libponyc on Linux

[SeanTAllen]

It's possible to use libponyc from projects other than ponyc. Using the
library gives you access to all the various compiler functionality. However,
it is hard to use the library because, you need to link in the correct libstdc++,
libblake, and LLVM. Figuring out what is the correct version of each is very
difficult.

With this change, a new library libponyc-standalone.a will be created as
part of the build process.

Applications like the "in pony documentation generator" that I am on will
be able to link to libponyc-standalone and pick up all it's required dependencies.

Windows, BSD, and macOS libraries are not created at this time but can be added
as needed in the future.

[ponylang-main]

Hi @SeanTAllen,

The changelog - added label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 3716.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.

[SeanTAllen]

@kulibali this creates the library but it ends up with linker errors.

[SeanTAllen]

If I use an ar script to include the same libraries, it seems to work.

[SeanTAllen]

There's a definite difference in the size of the libraries created:

-rw-r--r-- 1 root root 96049610 Feb 20 08:36 libponyc-full.a
-rw-r--r-- 1 root root 88715102 Feb 20 08:27 libponyc-standalone.a

So standalone has some linker errors for things missing, and full doesn't seem like standalone is missing something.

[SeanTAllen]

Ok so it isnt finding the .a correctly for stdlibc++.

[jemc]

Should we have a CI run that tests this new CMake target?

[SeanTAllen]

Eventually, in some form, yes

[SeanTAllen]

@kulibali I'm going to merge this later as I have other ponydoc related work to bring over the top of this. Feel free to comment and things can be fixed after the merge.

@jemc at the moment, ponydoc is the only user of this and will pick up breakage with nightly CI. When we get further along, I'll work out what is best for testing this.