Factor out common custom command generator

The Makefile, VS, and Xcode generators previously duplicated some custom
command line generation code.  Factor this out into a separate class
cmCustomCommandGenerator shared by all generators.

Source/cmCustomCommandGenerator.h

@@ -0,0 +1,37 @@
+/*============================================================================
+  CMake - Cross Platform Makefile Generator
+  Copyright 2000-2010 Kitware, Inc., Insight Software Consortium
+
+  Distributed under the OSI-approved BSD License (the "License");
+  see accompanying file Copyright.txt for details.
+
+  This software is distributed WITHOUT ANY WARRANTY; without even the
+  implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+  See the License for more information.
+============================================================================*/
+#ifndef cmCustomCommandGenerator_h
+#define cmCustomCommandGenerator_h
+
+#include "cmStandardIncludes.h"
+
+class cmCustomCommand;
+class cmMakefile;
+class cmLocalGenerator;
+
+class cmCustomCommandGenerator
+{
+  cmCustomCommand const& CC;
+  const char* Config;
+  cmMakefile* Makefile;
+  cmLocalGenerator* LG;
+  bool OldStyle;
+  bool MakeVars;
+public:
+  cmCustomCommandGenerator(cmCustomCommand const& cc, const char* config,
+                           cmMakefile* mf);
+  unsigned int GetNumberOfCommands() const;
+  std::string GetCommand(unsigned int c) const;
+  void AppendArguments(unsigned int c, std::string& cmd) const;
+};
+
+#endif