cmake: Fix the SOVERSION which needs to be 2 (#108)

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

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0d795cf..ff73d21 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -9,15 +9,18 @@
 
 project(libavif LANGUAGES C CXX VERSION 0.6.0)
 
-# SOVERSION scheme: CURRENT.AGE.REVISION
+# SOVERSION scheme: MAJOR.MINOR.PATCH
 #   If there was an incompatible interface change:
-#     Increment CURRENT. Set AGE and REVISION to 0
+#     Increment MAJOR. Set MINOR and PATCH to 0
 #   If there was a compatible interface change:
-#     Increment AGE. Set REVISION to 0
+#     Increment MINOR. Set PATCH to 0
 #   If the source code was changed, but there were no interface changes:
-#     Increment REVISION.
-set(LIBRARY_VERSION "2.0.0")
-set(LIBRARY_SOVERSION "0")
+#     Increment PATCH.
+set(LIBRARY_VERSION_MAJOR 2)
+set(LIBRARY_VERSION_MINOR 0)
+set(LIBRARY_VERSION_PATCH 0)
+set(LIBRARY_VERSION "${LIBRARY_VERSION_MAJOR}.${LIBRARY_VERSION_MINOR}.${LIBRARY_VERSION_PATCH}")
+set(LIBRARY_SOVERSION ${LIBRARY_VERSION_MAJOR})
 
 option(BUILD_SHARED_LIBS "Build shared avif library" ON)