The files are generated via flex and bison. Until now, this step was
performed when preparing the libc port. Unfortunately, the generated
files have subtle differences depending on the flex/bison versions
installed in the host. For example, the bison version number appears in
the generated code. This, in turn, breaks the hash mechanism of the
depot where a src/libc archive ends up being slightly different when
created on different hosts.
By moving the code generation to the build stage, the src/libc archive
merely contains the nslexer.l and nsparser.y source files but not the
generated files.
