Turn off %{source,header}_extension feature.

Turn off %{source,header}_extension feature.
* src/files.c (compute_exts_from_gf): Update.
(compute_exts_from_src): Update.
(output_files): CPP-out useless code.
* src/files.h: Remove {header,source}_extension extern declarations.
* src/reader.c (parse_dquoted_param): CPP-out.
(parse_header_extension_decl): Remove.
(parse_source_extension_decl): Remove.
(read_declarations): Remove cases tok_{hdrext,srcext}.
* src/lex.c (percent_table): Remove {header,source}_extension entries.
* src/lex.h (token_t): Remove tok_hdrext and tok_srcext.