fix appveyor mingw64 build
diff --git a/build/appveyor/MING-appveyor-build.bat b/build/appveyor/MING-appveyor-build.bat
index 6ebdb90..b37a95a 100644
--- a/build/appveyor/MING-appveyor-build.bat
+++ b/build/appveyor/MING-appveyor-build.bat
@@ -26,6 +26,7 @@
   -DCMAKE_MAKE_PROGRAM=/mingw%NORM_PLATFORM%/bin/mingw32-make ^
   -DCMAKE_C_COMPILER=/mingw%NORM_PLATFORM%/bin/gcc.exe ^
   -DCMAKE_CXX_COMPILER=/mingw%NORM_PLATFORM%/bin/g++.exe ^
+  -DOPENSSL_ROOT_DIR=/mingw%NORM_PLATFORM% ^
   -DWITH_PYTHON=OFF ^
   -DWITH_SHARED_LIB=OFF ^
   -DWITH_STATIC_LIB=ON