exec_program: Re-implement using KWSys Process

Drop use of cmSystemTools::RunCommand.  It used popen on UNIX
(equivalent to /bin/sh -c "$command") and direct CreateProcess calls on
Windows.  Implement equivalent behavior using the KWSys Process library.
Copy windows shortpath conversion logic from cmSystemTools::RunCommand.

Source/cmExecProgramCommand.h

@@ -57,6 +57,10 @@ public:
     }
 
   cmTypeMacro(cmExecProgramCommand, cmCommand);
+private:
+  static bool RunCommand(const char* command, std::string& output,
+                         int &retVal, const char* directory = 0,
+                         bool verbose = true);
 };
 
 #endif