Add of %source_extension and %header_extension which specify

Add of %source_extension and %header_extension which specify
the source or/and the header output file extension.
* src/files.c (compute_base_names): Remove initialisation of
src_extension and header_extension.
(compute_exts_from_gf): Update.
(compute_exts_from_src): Update.
(output_files): Update.
* src/reader.c (parse_header_extension_decl): New.
(parse_source_extension_decl): New.
(read_declarations): New case statements for the new tokens.
* src/lex.c (percent_table): Add entries for %source_extension
and %header_extension.
* src/lex.h (token_e): New tokens tok_hdrext and tok_srcext.