output: do not generate source files when late errors are caught

output: do not generate source files when late errors are caught

Reported by Alexandre Duret-Lutz as "second problem" in:
http://lists.gnu.org/archive/html/bug-bison/2013-09/msg00015.html

* bootstrap.conf: We need the "unlink" module.
* src/files.h, src/files.c (unlink_generated_sources): New.
* src/output.c: Use it.
* tests/output.at: Check the case of late errors.