Warn about output files that are generated by the skeletons and that

Warn about output files that are generated by the skeletons and that
conflict with other output files.
* data/glr.c: Don't generate the header file here when glr.cc does.
* src/files.c (file_names, file_names_count): New static globals.
(compute_output_file_names): Invoke output_file_name_check for files
not generated by the skeletons and remove existing checks.
(output_file_name_check): New function that warns about conflicting
output file names.
(output_file_names_free): Free file_names.
* src/files.h (output_file_name_check): Declare.
* src/scan-skel.l: Invoke output_file_name_check for files generated by
the skeletons.
* tests/output.at (AT_CHECK_CONFLICTING_OUTPUT): New.
(Conflicting output files): New tests.