Build OpenCV with FFmpeg on Linux

Due to the non-standard install location of FFmpeg during the AliceVision build OpenCV cannot find it. This patch adds FFmpeg to the BUILD_PREFIX of OpenCV to enable the FFmpeg features.

src/cmake/Dependencies.cmake

@@ -688,6 +688,9 @@ if(AV_BUILD_FFMPEG)
         BUILD_COMMAND $(MAKE) -j${AV_BUILD_DEPENDENCIES_PARALLEL}
         DEPENDS ${VPX_TARGET}
     )
+
+    set(FFMPEG_CMAKE_FLAGS -DCMAKE_PREFIX_PATH=${CMAKE_INSTALL_PREFIX};${CMAKE_PREFIX_PATH})
+
 endif()
 
 if(AV_BUILD_FLANN)
@@ -1004,7 +1007,7 @@ if(AV_BUILD_OPENCV)
             ${CMAKE_COMMAND} 
             ${CMAKE_CORE_BUILD_FLAGS}
             -DOPENCV_EXTRA_MODULES_PATH=${CMAKE_CURRENT_BINARY_DIR}/opencv_contrib/modules
-            ${ZLIB_CMAKE_FLAGS} ${TBB_CMAKE_FLAGS}
+            ${ZLIB_CMAKE_FLAGS} ${TBB_CMAKE_FLAGS} ${FFMPEG_CMAKE_FLAGS}
             ${TIFF_CMAKE_FLAGS} ${PNG_CMAKE_FLAGS} ${JPEG_CMAKE_FLAGS} ${LIBRAW_CMAKE_FLAGS}
             -DWITH_TBB=ON
             -DWITH_FFMPEG=${AV_BUILD_FFMPEG}