cmake: Build libavif as shared library by default

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>

CMakeLists.txt

@@ -8,6 +8,8 @@ if(CMAKE_CXX_COMPILER_ID MATCHES "GNU")
   add_definitions(-std=c99) # Enforce C99 for gcc
 endif()
 
+option(BUILD_SHARED_LIBS "Build shared libraries" ON)
+
 option(AVIF_CODEC_AOM "Use the AOM codec for encoding (and decoding if no other decoder is present)" OFF)
 option(AVIF_CODEC_DAV1D "Use the dav1d codec for decoding (overrides AOM decoding if also enabled)" OFF)
 option(AVIF_CODEC_RAV1E "Use the rav1e codec for encoding (overrides AOM encoding if also enabled)" OFF)