fix(*): [[fallthrough]] ==> /* fall-thru */

Older gcc compilers generate a warning when the attribute is used.
I found out that GCC 7 will not produce a warning if comments
such as /* fall-thru */ or /* FALLTHRU */ are used instead of the
attribute [[fallthrough]]

src/frontends/lean/parser.cpp

@@ -2511,7 +2511,7 @@ bool parser::parse_command_like() {
                 next();
                 break;
             }
-            [[fallthrough]];
+            /* fall-thru */
         default:
             throw parser_error("command expected", pos());
     }

src/library/mt_task_queue.cpp

@@ -303,7 +303,7 @@ void mt_task_queue::cancel_core(gtask const & t) {
     switch (get_state(t).load()) {
         case task_state::Waiting:
             m_waiting.erase(t);
-            [[fallthrough]];
+            /* fall-thru */
         case task_state::Created: case task_state::Queued:
             fail(t, std::make_exception_ptr(cancellation_exception()));
             handle_finished(t);

src/library/vm/vm.cpp

@@ -877,7 +877,7 @@ void vm_instr::serialize(serializer & s, std::function<name(unsigned)> const & i
         break;
     case opcode::BuiltinCases:
         s << idx2name(m_cases_idx);
-        [[fallthrough]];
+        /* fall-thru */
     case opcode::CasesN:
         s << m_npcs[0];
         for (unsigned j = 1; j < m_npcs[0] + 1; j++)

src/util/hash.cpp

@@ -41,17 +41,17 @@ unsigned hash_str(unsigned length, char const * str, unsigned init_value) {
     c += length;
     switch (len) {
         /* all the case statements fall through */
-    case 11:  c+=((unsigned)str[10] << 24);  [[fallthrough]];
-    case 10:  c+=((unsigned)str[9] << 16);   [[fallthrough]];
-    case 9 :  c+=((unsigned)str[8] << 8);    [[fallthrough]];
+    case 11:  c+=((unsigned)str[10] << 24);  /* fall-thru */
+    case 10:  c+=((unsigned)str[9] << 16);   /* fall-thru */
+    case 9 :  c+=((unsigned)str[8] << 8);    /* fall-thru */
         /* the first byte of c is reserved for the length */
-    case 8 :  b+=((unsigned)str[7] << 24);   [[fallthrough]];
-    case 7 :  b+=((unsigned)str[6] << 16);   [[fallthrough]];
-    case 6 :  b+=((unsigned)str[5] << 8);    [[fallthrough]];
-    case 5 :  b+=str[4];                     [[fallthrough]];
-    case 4 :  a+=((unsigned)str[3] << 24);   [[fallthrough]];
-    case 3 :  a+=((unsigned)str[2] << 16);   [[fallthrough]];
-    case 2 :  a+=((unsigned)str[1] << 8);    [[fallthrough]];
+    case 8 :  b+=((unsigned)str[7] << 24);   /* fall-thru */
+    case 7 :  b+=((unsigned)str[6] << 16);   /* fall-thru */
+    case 6 :  b+=((unsigned)str[5] << 8);    /* fall-thru */
+    case 5 :  b+=str[4];                     /* fall-thru */
+    case 4 :  a+=((unsigned)str[3] << 24);   /* fall-thru */
+    case 3 :  a+=((unsigned)str[2] << 16);   /* fall-thru */
+    case 2 :  a+=((unsigned)str[1] << 8);    /* fall-thru */
     case 1 :  a+=str[0];
         /* case 0: nothing left to add */
     }

src/util/hash.h

@@ -81,7 +81,7 @@ unsigned hash(unsigned n, H h, unsigned init_value = 31) {
 
         a += init_value;
         switch (n) {
-        case 2: b += h(1); [[fallthrough]];
+        case 2: b += h(1); /* fall-thru */
         case 1: c += h(0);
         }
         mix(a, b, c);

src/util/lp/lp_core_solver_base.cpp

@@ -350,7 +350,7 @@ column_is_dual_feasible(unsigned j) const {
     case upper_bound:
         std::cout << "upper_bound type should be switched to low_bound" << std::endl;
         lean_unreachable();
-        [[fallthrough]];
+        /* fall-thru */
     case free_column:
         return numeric_traits<X>::is_zero(m_d[j]);
     default: