Replace yyscan_t with another pointer type that we control.

This prevents the circuluar dependency between parser.gen.h and
lexer.gen.h.  Newer versions of bison add a prototype for yyparse() to
parser.gen.h that include the as-yet-undeclared yyscan_t type.

lexer.l

@@ -1,5 +1,8 @@
 %{
 #include "compile.h"
+
+struct lexer_param;
+
 #include "parser.gen.h"  /* Generated by bison. */
 
 #define YY_USER_ACTION                           \