cmake: allow FFmpeg linking on Android

details: https://github.com/opencv/opencv/pull/13726

In order to help CMake and PkgConfig find FFmpeg you need to:
1) export `PKG_CONFIG_LIBDIR=${SYSROOT}`
where `SYSROOT=${ANDROID_NDK}/sysroot`
2) Add `-DCMAKE_PREFIX_PATH=${FFMPEG_INSTALL_PREFIX}`
3) Append `-DCMAKE_FIND_ROOT_PATH=${FFMPEG_INSTALL_PREFIX}`

CMakeLists.txt

@@ -255,8 +255,8 @@ OCV_OPTION(WITH_EIGEN "Include Eigen2/Eigen3 support" (NOT CV_DISABLE_OPTIMIZATI
 OCV_OPTION(WITH_VFW "Include Video for Windows support (deprecated, consider using MSMF)" OFF
   VISIBLE_IF WIN32
   VERIFY HAVE_VFW)
-OCV_OPTION(WITH_FFMPEG "Include FFMPEG support" ON
-  VISIBLE_IF NOT ANDROID AND NOT IOS AND NOT WINRT
+OCV_OPTION(WITH_FFMPEG "Include FFMPEG support" (NOT ANDROID)
+  VISIBLE_IF NOT IOS AND NOT WINRT
   VERIFY HAVE_FFMPEG)
 OCV_OPTION(WITH_GSTREAMER "Include Gstreamer support" ON
   VISIBLE_IF NOT ANDROID AND NOT IOS AND NOT WINRT