Make decoding optional

Closes #249
diff --git a/CMakeLists.txt b/CMakeLists.txt
index bf67ae2..b791f54 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -271,7 +271,7 @@
 endif()
 
 if(NOT AVIF_CODEC_AOM AND NOT AVIF_CODEC_DAV1D AND NOT AVIF_CODEC_LIBGAV1)
-    message(FATAL_ERROR "libavif: No decoding library is enabled, bailing out.")
+    message(WARNING "libavif: No decoding library is enabled.")
 endif()
 
 add_library(avif ${AVIF_SRCS})