media-video/ffmpeg: add appkit USE flag

This flag controls a corresponding configure option, which previously
was left to autodetection, causing misdetection and compilation failure
for users having GNUstep installed.

Bug: https://bugs.gentoo.org/637482
Suggested-by: Alexis Ballier <aballier@gentoo.org>
Acked-by: Luca Barbato <lu_zero@gentoo.org>
Acked-by: Alexis Ballier <aballier@gentoo.org>
Package-Manager: Portage-2.3.17, Repoman-2.3.6

media-video/ffmpeg/ffmpeg-3.4.1.ebuild

@@ -78,6 +78,7 @@ FFMPEG_FLAG_MAP=(
 		speex:libspeex svg:librsvg vorbis:libvorbis
 		vpx:libvpx zvbi:libzvbi
 		# libavfilter options
+		appkit
 		bs2b:libbs2b chromaprint flite:libflite frei0r
 		fribidi:libfribidi fontconfig ladspa libass truetype:libfreetype
 		rubberband:librubberband zeromq:libzmq zimg:libzimg

media-video/ffmpeg/ffmpeg-9999.ebuild

@@ -78,6 +78,7 @@ FFMPEG_FLAG_MAP=(
 		speex:libspeex svg:librsvg vorbis:libvorbis
 		vpx:libvpx zvbi:libzvbi
 		# libavfilter options
+		appkit
 		bs2b:libbs2b chromaprint flite:libflite frei0r
 		fribidi:libfribidi fontconfig ladspa libass truetype:libfreetype
 		rubberband:librubberband zeromq:libzmq zimg:libzimg

media-video/ffmpeg/metadata.xml

@@ -7,6 +7,7 @@
 <use>
 	<flag name="amr">Enables Adaptive Multi-Rate Audio support</flag>
 	<flag name="amrenc">Enables Adaptive Multi-Rate Audio encoding support with <pkg>media-libs/vo-amrwbenc</pkg>.</flag>
+	<flag name="appkit">Enables Apple AppKit framework</flag>
 	<flag name="armv5te">Enables optimizations for armv5te processors.</flag>
 	<flag name="armv6">Enables optimizations for armv6 processors.</flag>
 	<flag name="armv6t2">Enables optimizations for armv6t2 processors.</flag>