cmSystemTools: Fix TrimWhitespace for non-ascii strings (#15735)

Since commit v2.8.11~59^2 (cmSystemTools: Generalize TrimWhitespace to
all whitespace, 2013-03-27) we incorrectly use `c <= ' '` to determine
if `c` is a whitespace character.  With a signed `char` type UTF-8
encoded strings may be truncated because values above 0x7f appear
negative and therefore less than 0x20.  Use `isspace(c)` instead.

Source/cmSystemTools.cxx

@@ -233,13 +233,13 @@ std::string cmSystemTools::HelpFileName(std::string name)
 std::string cmSystemTools::TrimWhitespace(const std::string& s)
 {
   std::string::const_iterator start = s.begin();
-  while(start != s.end() && *start <= ' ')
+  while (start != s.end() && cm_isspace(*start))
     ++start;
   if (start == s.end())
     return "";
 
   std::string::const_iterator stop = s.end()-1;
-  while(*stop <= ' ')
+  while (cm_isspace(*stop))
     --stop;
   return std::string(start, stop+1);
 }