examples: include the generated header

* examples/c/bistromathic/parse.y, examples/c/lexcalc/parse.y,
* examples/c/reccalc/parse.y: here.
Add some comments.

* src/parse-gram.y (api_version): Pull out of handle_require.
Bump to 3.7.

examples/c/lexcalc/parse.y

@@ -19,6 +19,9 @@
 #include <stdlib.h> // getenv.
 }
 
+// Include the header in the implementation rather than duplicating it.
+%define api.header.include {"parse.h"}
+
 // Don't share global variables between the scanner and the parser.
 %define api.pure full