c: Support C99-style Unicode character escapes
* x-c.c: Include assert.h and po-charset.h.
(P7_QUOTES, P7_QUOTE, P7_NEWLINE): Redefine as a negative integer.
(P7_EOF, P7_STRING_END): New definitions.
(UNICODE): New macro.
(IS_UNICODE): New macro.
(UNICODE_VALUE): New macro.
(phase7_getc): Recognize "\unnnn" and "\Unnnnnnnn".
(phase5_get): Use mixed_string_buffer for parse string literal.
