Add cmGeneratorExpression::Split() API.

It can split a string like

 "A;$<1:B>;$<1:C>;D;E;$<1:F;G;H>;$<1:I>;J"

into

 "A" "$<1:B>" "$<1:C>" "D" "E" "$<1:F;G;H>" "$<1:I>" "J"

Source/cmGeneratorExpression.cxx

@@ -250,6 +250,67 @@ static std::string stripExportInterface(const std::string &input,
   return result;
 }
 
+//----------------------------------------------------------------------------
+void cmGeneratorExpression::Split(const std::string &input,
+                                  std::vector<std::string> &output)
+{
+  std::string::size_type pos = 0;
+  std::string::size_type lastPos = pos;
+  while((pos = input.find("$<", lastPos)) != input.npos)
+    {
+    std::string part = input.substr(lastPos, pos - lastPos);
+    std::string preGenex;
+    if (!part.empty())
+      {
+      std::string::size_type startPos = input.rfind(";", pos);
+      if (startPos != pos - 1 && startPos >= lastPos)
+        {
+        part = input.substr(lastPos, startPos - lastPos);
+        preGenex = input.substr(startPos + 1, pos - startPos - 1);
+        }
+      cmSystemTools::ExpandListArgument(part.c_str(), output);
+      }
+    pos += 2;
+    int nestingLevel = 1;
+    const char *c = input.c_str() + pos;
+    const char * const cStart = c;
+    for ( ; *c; ++c)
+      {
+      if(c[0] == '$' && c[1] == '<')
+        {
+        ++nestingLevel;
+        ++c;
+        continue;
+        }
+      if(c[0] == '>')
+        {
+        --nestingLevel;
+        if (nestingLevel == 0)
+          {
+          break;
+          }
+        }
+      }
+    for ( ; *c; ++c)
+      {
+      // Capture the part after the genex and before the next ';'
+      if(c[0] == ';')
+        {
+        --c;
+        break;
+        }
+      }
+    const std::string::size_type traversed = (c - cStart) + 1;
+    output.push_back(preGenex + "$<" + input.substr(pos, traversed));
+    pos += traversed;
+    lastPos = pos;
+    }
+  if (lastPos < input.size())
+    {
+    cmSystemTools::ExpandListArgument(input.substr(lastPos), output);
+    }
+}
+
 //----------------------------------------------------------------------------
 std::string cmGeneratorExpression::Preprocess(const std::string &input,
                                               PreprocessContext context)

Source/cmGeneratorExpression.h

@@ -59,6 +59,9 @@ public:
   static std::string Preprocess(const std::string &input,
                                 PreprocessContext context);
 
+  static void Split(const std::string &input,
+                    std::vector<std::string> &output);
+
 private:
   cmGeneratorExpression(const cmGeneratorExpression &);
   void operator=(const cmGeneratorExpression &);