BUILD.md not present in tarball

[Ryan Carsten Schmidt]

README.md says:

re2c/README.md

Lines 50 to 51 in ece4bb7

 See BUILD.md or the [online docs](https://re2c.org/build/build.html) for 
 instructions how to build re2c. 

but there is no file BUILD.md in the 3.1 tarball. It's in the repository, so presumably you just need to tell whatever part of the build system creates the tarball to include that file.

[Ulya Trofimovich]

Thanks! Fixed in 3841e83.