ffmpeg[234]: Add an ssl optional group to only choice gnutls or openssl option

At least multimedia/ffmpeg4 does not support to be built with both `gnutls' and
`openssl'.  Introduce an `ssl' optional group in order to select only `gnutls'
or `openssl' option.

Noticed by John D. Baker on pkgsrc-users@:

 <https://mail-index.NetBSD.org/pkgsrc-users/2018/09/25/msg027482.html>

Thanks to John for spotting that and <gdt> for review!

Index: ffmpeg2/options.mk
===================================================================
RCS file: /cvsroot/pkgsrc/multimedia/ffmpeg2/options.mk,v
retrieving revision 1.13
diff -u -p -r1.13 options.mk
--- ffmpeg2/options.mk	20 Sep 2017 08:21:17 -0000	1.13
+++ ffmpeg2/options.mk	25 Sep 2018 19:43:09 -0000
@@ -3,8 +3,12 @@
 # Global and legacy options
 
 PKG_OPTIONS_VAR=	PKG_OPTIONS.ffmpeg2
-PKG_SUPPORTED_OPTIONS=	ass doc faac fdk-aac fontconfig freetype gnutls lame \
-			libvpx opencore-amr openssl rtmp theora vorbis x11 x264 \
+
+PKG_OPTIONS_OPTIONAL_GROUPS=	ssl
+PKG_OPTIONS_GROUP.ssl=		gnutls openssl
+
+PKG_SUPPORTED_OPTIONS=	ass doc faac fdk-aac fontconfig freetype lame \
+			libvpx opencore-amr rtmp theora vorbis x11 x264 \
 			x265 xcb xvid
 PKG_SUGGESTED_OPTIONS=	lame ass freetype fontconfig libvpx openssl \
 			theora vorbis x11 x264 xvid
Index: ffmpeg3/options.mk
===================================================================
RCS file: /cvsroot/pkgsrc/multimedia/ffmpeg3/options.mk,v
retrieving revision 1.13
diff -u -p -r1.13 options.mk
--- ffmpeg3/options.mk	25 Sep 2018 09:46:09 -0000	1.13
+++ ffmpeg3/options.mk	25 Sep 2018 19:43:09 -0000
@@ -3,8 +3,12 @@
 # Global and legacy options
 
 PKG_OPTIONS_VAR=	PKG_OPTIONS.ffmpeg3
-PKG_SUPPORTED_OPTIONS=	ass bluray doc fdk-aac fontconfig freetype gnutls \
-			lame libvpx opencore-amr openssl opus rpi rtmp \
+
+PKG_OPTIONS_OPTIONAL_GROUPS=	ssl
+PKG_OPTIONS_GROUP.ssl=		gnutls openssl
+
+PKG_SUPPORTED_OPTIONS=	ass bluray doc fdk-aac fontconfig freetype \
+			lame libvpx opencore-amr opus rpi rtmp \
 			tesseract theora vorbis x11 x264 x265 xcb xvid
 PKG_SUGGESTED_OPTIONS=	lame ass bluray freetype fontconfig libvpx openssl \
 			theora vorbis x11 x264 xvid
Index: ffmpeg4/options.mk
===================================================================
RCS file: /cvsroot/pkgsrc/multimedia/ffmpeg4/options.mk,v
retrieving revision 1.3
diff -u -p -r1.3 options.mk
--- ffmpeg4/options.mk	25 Sep 2018 09:39:56 -0000	1.3
+++ ffmpeg4/options.mk	25 Sep 2018 19:43:09 -0000
@@ -3,8 +3,12 @@
 # Global and legacy options
 
 PKG_OPTIONS_VAR=	PKG_OPTIONS.ffmpeg4
-PKG_SUPPORTED_OPTIONS=	ass bluray doc fdk-aac fontconfig freetype gnutls \
-			lame libvpx opencore-amr openssl opus rpi rtmp \
+
+PKG_OPTIONS_OPTIONAL_GROUPS=	ssl
+PKG_OPTIONS_GROUP.ssl=		gnutls openssl
+
+PKG_SUPPORTED_OPTIONS=	ass bluray doc fdk-aac fontconfig freetype \
+			lame libvpx opencore-amr opus rpi rtmp \
 			tesseract theora vorbis x11 x264 x265 xcb xvid
 PKG_SUGGESTED_OPTIONS=	lame ass bluray freetype fontconfig libvpx openssl \
 			theora vorbis x11 x264 xvid