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/core/src/persistence.cpp

@@ -114,7 +114,7 @@ cv::string cv::FileStorage::getDefaultObjectName(const string& _filename)
 
 namespace cv
 {
-
+#ifndef ANDROID //unsuported wcstombs on android
 string fromUtf16(const WString& str)
 {
     cv::AutoBuffer<char> _buf(str.size()*4 + 1);
@@ -138,7 +138,7 @@ WString toUtf16(const string& str)
     buf[sz] = '\0';
     return WString(buf);
 }
-
+#endif
 }
 
 typedef struct CvGenericHash