CMake: Handle indeterminate FFMpeg HWAccel status

If we don't have a version number and can't conclusively determine
whether hwaccel is available, but the user hasn't disabled it
explicitly, just don't report its status at all. Better than
reporting it's disabled when that may not be true at all.

src/CMakeLists.txt

@@ -327,11 +327,17 @@ endif()
 # Include in feature summary
 if(USE_HW_ACCEL AND HAVE_HW_ACCEL)
   set(FFMPEG_HARDWARE_ACCELERATION TRUE)
+elseif(USE_HW_ACCEL)
+  set(FFMPEG_HARDWARE_ACCELERATION "NOTFOUND")
 else()
   set(FFMPEG_HARDWARE_ACCELERATION FALSE)
 endif()
-set(_hwaccel_help "GPU-accelerated routines (FFmpeg 3.4+)")
-add_feature_info("FFmpeg hwaccel" FFMPEG_HARDWARE_ACCELERATION ${_hwaccel_help})
+
+# Report HWAccel status, unless it's indeterminate
+if (NOT FFMPEG_HARDWARE_ACCELERATION STREQUAL "NOTFOUND")
+  set(_hwaccel_help "GPU-accelerated routines (FFmpeg 3.4+)")
+  add_feature_info("FFmpeg hwaccel" FFMPEG_HARDWARE_ACCELERATION ${_hwaccel_help})
+endif()
 
 ################### OPENMP #####################
 # Check for OpenMP (used for multi-core processing)