examples: reccalc: compile cleanly in C99

See https://trac.macports.org/ticket/59927.

* examples/c/reccalc/parse.y: C99 does not allow multiple typedefs.

examples/c/reccalc/parse.y

@@ -133,6 +133,9 @@ exp:
 %%
 // Epilogue (C code).
 
+// We already defined yyscan_t, don't let scan.h define it again.
+#define YY_TYPEDEF_YY_SCANNER_T
+typedef void* yyscan_t;
 #include "scan.h"
 
 result