fix ffmpeg cmake scripts not working with cmake 3.8

dlib/CMakeLists.txt

@@ -838,7 +838,7 @@ if (NOT TARGET dlib)
          include(cmake_utils/find_ffmpeg.cmake)
          if (FFMPEG_FOUND)
             list (APPEND dlib_needed_public_includes ${FFMPEG_INCLUDE_DIRS})
-            list (APPEND dlib_needed_public_libraries ${FFMPEG_LINK_LIBRARIES})
+            list (APPEND dlib_needed_public_libraries PkgConfig::FFMPEG)
             enable_preprocessor_switch(DLIB_USE_FFMPEG)
          else()
             set(DLIB_USE_FFMPEG OFF CACHE BOOL ${DLIB_USE_FFMPEG_STR} FORCE )

dlib/cmake_utils/find_ffmpeg.cmake

@@ -1,3 +1,5 @@
+cmake_minimum_required(VERSION 3.8.0)
+
 message(STATUS "Searching for FFMPEG/LIBAV")
 find_package(PkgConfig)
 if (PkgConfig_FOUND)
@@ -24,4 +26,4 @@ if (PkgConfig_FOUND)
 else()
     message(STATUS "PkgConfig could not be found, FFMPEG won't be available")
     set(FFMPEG_FOUND 0)
-endif()
+endif()