fix C90 compliance

* data/glr.c, src/graphviz.h, src/ielr.c, src/scan-gram.l,
* src/system.h, tests/actions.at, tests/glr-regression.at: Do not
use // comments.
Do not introduce variables after statements.
Provide "main" with a return value.

src/system.h

@@ -51,7 +51,7 @@
 typedef size_t uintptr_t;
 # endif
 
-// Version mismatch.
+/* Version mismatch. */
 # define EX_MISMATCH 63
 
 /*---------.