lib/ffmpeg/meson.build: move libavutil helpers into separate library

src/lib/ffmpeg/meson.build

@@ -13,6 +13,29 @@ else
 endif
 conf.set('HAVE_LIBAVFILTER', libavfilter_dep.found())
 
+if not libavutil_dep.found()
+  ffmpeg_util_dep = dependency('', required: false)
+  ffmpeg_dep = dependency('', required: false)
+  subdir_done()
+endif
+
+ffmpeg_util = static_library(
+  'ffmpeg_util',
+  'Interleave.cxx',
+  'Error.cxx',
+  include_directories: inc,
+  dependencies: [
+    libavutil_dep,
+  ],
+)
+
+ffmpeg_util_dep = declare_dependency(
+  link_with: ffmpeg_util,
+  dependencies: [
+    libavutil_dep,
+  ],
+)
+
 if not enable_ffmpeg
   ffmpeg_dep = dependency('', required: false)
   subdir_done()
@@ -36,10 +59,10 @@ ffmpeg = static_library(
   ffmpeg_sources,
   include_directories: inc,
   dependencies: [
+    ffmpeg_util_dep,
     libavformat_dep,
     libavcodec_dep,
     libavfilter_dep,
-    libavutil_dep,
     log_dep,
   ],
 )
@@ -47,9 +70,9 @@ ffmpeg = static_library(
 ffmpeg_dep = declare_dependency(
   link_with: ffmpeg,
   dependencies: [
+    ffmpeg_util_dep,
     libavformat_dep,
     libavcodec_dep,
     libavfilter_dep,
-    libavutil_dep,
   ],
 )