cmGeneratorExpression: Improve parsing in StripEmptyListElements

The char-by-char parsing causes lots of reallocations which shouldn't be
necessary. To improve this, fast-path strings without a semicolon,
reserve space in the result, and insert into the result in chunks.

Source/cmGeneratorExpression.cxx

@@ -157,17 +157,24 @@ cmCompiledGeneratorExpression::~cmCompiledGeneratorExpression()
 std::string cmGeneratorExpression::StripEmptyListElements(
                                                     const std::string &input)
 {
+  if (input.find(';') == input.npos)
+    {
+    return input;
+    }
   std::string result;
+  result.reserve(input.size());
 
   const char *c = input.c_str();
+  const char *last = c;
   bool skipSemiColons = true;
   for ( ; *c; ++c)
     {
-    if(c[0] == ';')
+    if(*c == ';')
       {
       if(skipSemiColons)
         {
-        continue;
+        result.append(last, c - last);
+        last = c + 1;
         }
       skipSemiColons = true;
       }
@@ -175,8 +182,8 @@ std::string cmGeneratorExpression::StripEmptyListElements(
       {
       skipSemiColons = false;
       }
-    result += *c;
     }
+  result.append(last);
 
   if (!result.empty() && *(result.end() - 1) == ';')
     {