Re-word comment on unreachable return statements

Do not use the word "warning" in the comment because CTest launchers
match it and CTestCustom.cmake.in does not suppress it.

Source/cmCommandArgumentLexer.cxx

@@ -1069,7 +1069,7 @@ case YY_STATE_EOF(NOESCAPES):
                         "fatal flex scanner internal error--no action found" );
         } /* end of action switch */
                 } /* end of scanning one token */
-return 0; /* this should not happend but it should silence a warning */
+return 0; /* this should not happen but it quiets some compilers */
 } /* end of cmCommandArgument_yylex */
 
 /* yy_get_next_buffer - try to read in a new buffer

Source/cmDependsJavaLexer.cxx

@@ -1591,7 +1591,7 @@ case YY_STATE_EOF(string):
       "fatal flex scanner internal error--no action found" );
   } /* end of action switch */
     } /* end of scanning one token */
-return 0; /* this should not happen but it silences a warning*/
+return 0; /* this should not happen but it quiets some compilers */
 } /* end of cmDependsJava_yylex */
 
 /* yy_get_next_buffer - try to read in a new buffer

Source/cmExprLexer.cxx

@@ -976,7 +976,7 @@ case YY_STATE_EOF(INITIAL):
       "fatal flex scanner internal error--no action found" );
   } /* end of action switch */
     } /* end of scanning one token */
-return 0; /* this should not happen but it silences a warning*/
+return 0; /* this should not happen but it quiets some compilers */
 } /* end of cmExpr_yylex */
 
 /* yy_get_next_buffer - try to read in a new buffer

Source/cmSystemTools.cxx

@@ -1617,7 +1617,7 @@ long copy_data(struct archive *ar, struct archive *aw)
       }
     }
 #if !defined(__clang__)
-  return r; /* this should not happen but it silences a warning */
+  return r; /* this should not happen but it quiets some compilers */
 #endif
 }