cmExprLexer: Treat newline as whitespace explicitly

Newlines matched no rule, so they reached flex's default rule.  It
discards them only because cmStandardLexer.h defines ECHO as empty, and
`flex -s` warns "default rule can be matched".  Add the newline to the
whitespace rule to make the existing behavior of math(EXPR) explicit.

Source/LexerParser/cmExprLexer.cxx

@@ -559,7 +559,7 @@ struct yy_trans_info
 	};
 static const flex_int16_t yy_accept[29] =
     {   0,
-        0,    0,   19,   17,    1,   18,    8,   10,   15,   16,
+        0,    0,   19,   17,    1,    1,    8,   10,   15,   16,
         6,    4,    5,    7,    2,    2,   17,   17,   11,    9,
        12,    2,    0,   13,   14,    3,    3,    0
     } ;
@@ -987,6 +987,7 @@ do_action:	/* This label is used only to access EOF actions. */
 			goto yy_find_action;
 
 case 1:
+/* rule 1 can match eol */
 YY_RULE_SETUP
 {}
 	YY_BREAK

Source/LexerParser/cmExprLexer.in.l

@@ -42,7 +42,7 @@ Modify cmExprLexer.cxx:
 %pointer
 
 %%
-[ \t]  {}
+[ \t\n]  {}
 
 [0-9][0-9]* { yylvalp->Number = std::stoll(yytext, nullptr, 10); return exp_NUMBER; }
 0[xX][0-9a-fA-F][0-9a-fA-F]* {  yylvalp->Number = std::stoll(yytext, nullptr, 16); return exp_NUMBER; }