decklink: Add option to disable the plugin

Add an option to the cmake build system to disable the plugin. This is
needed on arm, as the plugin uses x86-intrinsics.

plugins/decklink/linux/CMakeLists.txt

@@ -1,5 +1,10 @@
 project(linux-decklink)
 
+if(DISABLE_DECKLINK)
+	message(STATUS "decklink plugin disabled")
+	return()
+endif()
+
 set(linux-decklink-sdk_HEADERS
 	decklink-sdk/DeckLinkAPI.h
 	decklink-sdk/DeckLinkAPIConfiguration.h

plugins/decklink/mac/CMakeLists.txt

@@ -1,5 +1,10 @@
 project(mac-decklink)
 
+if(DISABLE_DECKLINK)
+	message(STATUS "decklink plugin disabled")
+	return()
+endif()
+
 find_library(COREFOUNDATION CoreFoundation)
 
 include_directories(${COREFOUNDATION})

plugins/decklink/win/CMakeLists.txt

@@ -1,5 +1,10 @@
 project(win-decklink)
 
+if(DISABLE_DECKLINK)
+	message(STATUS "decklink plugin disabled")
+	return()
+endif()
+
 include(IDLFileHelper)
 
 set(win-decklink-sdk_IDLS