Configure reentrant bison/flex (#789)

Make the parser reentrant, working towards #769 and the ability to load other packages cleanly.

executable_semantics/syntax/parse_and_lex_context.h

@@ -40,8 +40,9 @@ class ParseAndLexContext {
 }  // namespace Carbon
 
 // Gives flex the yylex prototype we want.
-#define YY_DECL \
-  Carbon::Parser::symbol_type yylex(Carbon::ParseAndLexContext& context)
+#define YY_DECL                                         \
+  Carbon::Parser::symbol_type yylex(yyscan_t yyscanner, \
+                                    Carbon::ParseAndLexContext& context)
 
 // Declares yylex for the parser's sake.
 YY_DECL;