How to build `alex` from git sources? Instructions in `README.md` are broken.

https://github.com/simonmar/alex/blob/e4843f22818451299acd3620042b151ad2294ace/README.md#L51-L57
Nope:
```
$ cabal install
Error: cabal: sdist of alex-3.2.7: filepath wildcard 'src/Parser.y.boot' does
not match any files.
```

https://github.com/simonmar/alex/blob/e4843f22818451299acd3620042b151ad2294ace/README.md#L59-L66
Again, sorry, no (even with the correct tarball location):
```
$ make sdist
...
Wrote tarball sdist to
./dist-newstyle/sdist/alex-3.2.7.tar.gz
...
$ cabal install ./dist-newstyle/sdist/alex-3.2.7.tar.gz
Error: cabal: Unknown target './dist-newstyle/sdist/alex-3.2.7.tar.gz'.
The package alex has no file target 'dist-newstyle/sdist/alex-3.2.7.tar.gz'.
```

The `Makefile` goals work though, e.g., I can run the tests locally with `make sdist-test`.

Review the build-from-git sections in README.md at the linked lines and compare them with the working Makefile goals, especially make sdist-test. Reproduce the documented cabal commands, then update the instructions so they describe a currently valid workflow and verify it against the reported source-build path.

Written by the indexing model from the issue text.