ENH: Generalize exe implib dir creation for VS

In VS 7,8,9 executable targets we generate a build event to create the
output directory for the import library in case the executable marks
symbols with dllexport (VS forgets to create this directory).  This
generalizes computation of the custom command line to support future use
with other VS versions.

Source/cmLocalVisualStudioGenerator.h

@@ -19,6 +19,8 @@
 
 #include "cmLocalGenerator.h"
 
+#include <cmsys/auto_ptr.hxx>
+
 class cmSourceFile;
 class cmSourceGroup;
 
@@ -35,6 +37,10 @@ public:
   virtual ~cmLocalVisualStudioGenerator();
 protected:
 
+  /** Construct a custom command to make exe import lib dir.  */
+  cmsys::auto_ptr<cmCustomCommand>
+  MaybeCreateImplibDir(cmTarget& target, const char* config);
+
   /** Construct a script from the given list of command lines.  */
   std::string ConstructScript(const cmCustomCommandLines& commandLines,
                               const char* workingDirectory,