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.

tests/output.at

@@ -43,7 +43,7 @@ done
 ]AT_DATA([$1],
 [$2[
 %%
-foo: {};
+foo: %empty {};
 ]])[
 
 ]AT_BISON_CHECK([$3 $1], [$5], [], [ignore])[
@@ -126,6 +126,14 @@ AT_CHECK_OUTPUT([foo.y], [%type <foo> useless],
                 [foo.dot foo.output foo.xml],
                 [1])
 
+# Do not generate code when there are late errors (even warnings as
+# errors).
+AT_CHECK_OUTPUT([foo.y], [%define useless],
+                [--defines --graph --xml --report=all -Wall -Werror],
+                [foo.dot foo.output foo.xml],
+                [1])
+
+
 ## ------------ ##
 ## C++ output.  ##
 ## ------------ ##