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/graphviz.h

@@ -24,38 +24,48 @@
 
 #include "state.h"
 
-/// Begin a Dot graph.
-/// \param fout   output stream.
+/** Begin a Dot graph.
+ *
+ * \param fout   output stream.
+ */
 void start_graph (FILE *fout);
 
-/// Output a Dot node.
-/// \param id     identifier of the node
-/// \param label  human readable label of the node (no Dot escaping needed).
-/// \param fout   output stream.
+/** Output a Dot node.
+ *
+ * \param id     identifier of the node
+ * \param label  human readable label of the node (no Dot escaping needed).
+ * \param fout   output stream.
+ */
 void output_node (int id, char const *label, FILE *fout);
 
-/// Output a Dot edge.
-/// \param source       id of the source node
-/// \param destination  id of the target node
-/// \param label        human readable label of the edge
-///                     (no Dot escaping needed).  Can be 0.
-/// \param style        Dot style of the edge (e.g., "dotted" or "solid").
-/// \param fout         output stream.
+/** Output a Dot edge.
+ * \param source       id of the source node
+ * \param destination  id of the target node
+ * \param label        human readable label of the edge
+ *                     (no Dot escaping needed).  Can be 0.
+ * \param style        Dot style of the edge (e.g., "dotted" or "solid").
+ * \param fout         output stream.
+ */
 void output_edge (int source, int destination, char const *label,
                   char const *style, FILE *fout);
 
-/// Output a reduction.
-/// \param s            current state
-/// \param reds         the set of reductions
-/// \param fout         output stream.
+/** Output a reduction.
+ * \param s            current state
+ * \param reds         the set of reductions
+ * \param fout         output stream.
+ */
 void output_red (state const *s, reductions const *reds, FILE *fout);
 
-/// End a Dot graph.
-/// \param fout  output stream.
+/** End a Dot graph.
+ *
+ * \param fout  output stream.
+ */
 void finish_graph (FILE *fout);
 
-/// Escape a lookahead token.
-/// \param name         the token.
+/** Escape a lookahead token.
+ *
+ * \param name         the token.
+ */
 char const *escape (char const *name);
 
 #endif /* ! GRAPHVIZ_H_ */

src/ielr.c

@@ -758,8 +758,8 @@ ielr_compute_state (bitsetv follow_kernel_items, bitsetv always_follows,
                     if (!bitset_empty_p (lookaheads[i]))
                       break;
                   }
-                // bitset_equal_p uses the size of the first argument, so
-                // lookaheads[i] must be the second argument.
+                /* bitset_equal_p uses the size of the first argument,
+                   so lookaheads[i] must be the second argument.  */
                 else if (!bitset_equal_p ((*this_isocorep)->lookaheads[i],
                                           lookaheads[i]))
                   break;
@@ -1187,8 +1187,8 @@ ielr (void)
   free (to_state);
   if (lr_type == LR_TYPE__CANONICAL_LR)
     {
-      // Reduction lookaheads are computed in ielr_split_states above but are
-      // timed as part of phase 4.
+      /* Reduction lookaheads are computed in ielr_split_states above
+         but are timed as part of phase 4. */
       set_goto_map ();
     }
   else

src/scan-gram.l

@@ -883,7 +883,7 @@ unexpected_end (boundary start, char const *msgid, char const *token_end)
   loc.start = start;
   loc.end = scanner_cursor;
   token_end = quote (token_end);
-  // Instead of '\'', display "'".
+  /* Instead of '\'', display "'".  */
   if (!strcmp (token_end, "'\\''"))
     token_end = "\"'\"";
   complain_at (loc, _(msgid), token_end);

src/system.h

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