adding some small changes to support android build,

mainly cmake stuff, but in persistance the wcstombs
is not supported on android, and in
sift.cpp there was and ifdef that affected arm and this
causes undefined symbols on android.

modules/features2d/src/sift.cpp

@@ -48,7 +48,15 @@
 
 #include "precomp.hpp"
 
-#ifndef __arm__
+#ifdef __arm__
+#define ARM_NO_SIFT
+#endif
+
+#ifdef ANDROID
+#undef  ARM_NO_SIFT
+#endif //ANDROID
+
+#ifndef ARM_NO_SIFT
 
 #include <iostream>
 #include <limits>
@@ -175,6 +183,7 @@ public:
   typedef Keypoints::iterator       KeypointsIter ;      ///< Keypoint list iter datatype
   typedef Keypoints::const_iterator KeypointsConstIter ; ///< Keypoint list const iter datatype
 
+#undef _S
   /** @brief Constructors and destructors */
   /*@{*/
   Sift(const pixel_t* _im_pt, int _width, int _height,