-Werror=implicit-fallthrough= (GCC 7)

Change-Id: I2e9ab956fc1e59d09b7409333f18230e34b6a9b7

soltools/cpp/_eval.c

@@ -319,7 +319,7 @@ long
                     }
                     continue;
                 }
-                /* flow through */
+                /* fall through */
 
                 /* plain binary */
             case EQ:

soltools/cpp/_lex.c

@@ -492,6 +492,7 @@ continue2:
 
                 case S_STNL:
                     error(ERROR, "Unterminated string or char const");
+                    /* fall through */
                 case S_NL:
                     tp->t = ip;
                     tp->type = NL;
@@ -516,6 +517,7 @@ continue2:
                 case S_EOFCOM:
                     error(WARNING, "EOF inside comment");
                     --ip;
+                    /* fall through */
                 case S_COMMENT:
                     if (!Cflag)
                     {