EvalIf: Consider any whitespace in variable names an error

Not only are ' ' and '\t' are considered bad in variable names used in
if statements, actually any whitespace including '\n' is considered
illegal. Fix that by testing for anything matching `isspace`.

This fixes 'testcase/ifdef_ret_in_arg.mk'.

Signed-off-by: Matthias Maennich <maennich@google.com>

src/eval.cc

diff --git a/src/eval.cc b/src/eval.cc
index a58dcda..7aa82fb 100644
--- a/src/eval.cc
+++ b/src/eval.cc
@@ -16,6 +16,7 @@
 
 #include "eval.h"
 
+#include <ctype.h>
 #include <errno.h>
 #include <pthread.h>
 #include <stdio.h>
@@ -574,8 +575,10 @@
       string var_name;
       stmt->lhs->Eval(this, &var_name);
       Symbol lhs = Intern(TrimRightSpace(var_name));
-      if (lhs.str().find_first_of(" \t") != string::npos)
+      if (const auto& s = lhs.str();
+          std::find_if(s.begin(), s.end(), ::isspace) != s.end()) {
         Error("*** invalid syntax in conditional.");
+      }
       Var* v = LookupVarInCurrentScope(lhs);
       v->Used(this, lhs);
       is_true = (v->String().empty() == (stmt->op == CondOp::IFNDEF));