c: Interpret string literals lazily
* x-c.c (P7_EOF, P7_STRING_END, P7_QUOTES, P7_QUOTE, P7_NEWLINE)
(UNICODE, IS_UNICODE, UNICODE_VALUE): Remove.
(phase7_get): Remove.
(phase7_ungetc): Remove.
(phase5_get): Use 'phase3_get' directly to extract string
literals; use 'arglist_parser_remember_literal' instead of
'arglist_parser_remember'.
(literalstring_parse): New function.
(literalstring_c): New variable.
(extract_parenthesized): Remove the
'xgettext_current_source_encoding' setting to prevent encoding
conversion around 'arglist_parser_done'.
* x-c.h (SCANNERS_C): Register 'literalstring_c' as a
literalstring_parser.
(literalstring_c): New variable declaration.
