tccpp.c: parse flag to accept stray \

This adds a PARSE_FLAG_ACCEPT_STRAYS parse flag to accept stray
backslashes in the source code, and uses it for pure preprocessing.

For absolutely correct behaviour of # stringification, we need to use
this flag when parsing macro definitions and in macro arguments, as
well; this patch does not yet do so. The test case for that is something
like

    #define STRINGIFY2(x) #x
    #define STRINGIFY(x) STRINGIFY2(x)

    STRINGIFY(\n)

which should produce "\n", not a parse error or "\\n".

See http://lists.nongnu.org/archive/html/tinycc-devel/2015-05/msg00002.html

tcc.h

@@ -1152,6 +1152,7 @@ ST_DATA TokenSym **table_ident;
                                         returned at eof */
 #define PARSE_FLAG_ASM_FILE 0x0008 /* we processing an asm file: '#' can be used for line comment, etc. */
 #define PARSE_FLAG_SPACES     0x0010 /* next() returns space tokens (for -E) */
+#define PARSE_FLAG_ACCEPT_STRAYS 0x0020 /* next() returns '\\' token */
 
 ST_FUNC TokenSym *tok_alloc(const char *str, int len);
 ST_FUNC char *get_tok_str(int v, CValue *cv);