cmake: fix version in soname

CMakeLists.txt

@@ -35,6 +35,8 @@ set(LT_AGE      4)
 set(CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/cmake")
 include(Version)
 
+math(EXPR LT_SOVERSION "${LT_CURRENT} - ${LT_AGE}")
+set(LT_VERSION "${LT_SOVERSION}.${LT_AGE}.${LT_REVISION}")
 set(PACKAGE_VERSION     "${PROJECT_VERSION}")
 HexVersion(PACKAGE_VERSION_NUM ${PROJECT_VERSION_MAJOR} ${PROJECT_VERSION_MINOR} ${PROJECT_VERSION_PATCH})
 

lib/CMakeLists.txt

@@ -27,8 +27,7 @@ add_library(nghttp2 SHARED ${NGHTTP2_SOURCES})
 #target_link_libraries(nghttp2 ...)
 set_target_properties(nghttp2 PROPERTIES
   COMPILE_FLAGS "${WARNCFLAGS} ${EXTRACFLAG}"
-  VERSION ${LT_CURRENT}.${LT_REVISION}.${LT_AGE}
-  SOVERSION ${LT_CURRENT})
+  VERSION ${LT_VERSION} SOVERSION ${LT_SOVERSION})
 
 install(TARGETS nghttp2
   LIBRARY DESTINATION "${CMAKE_INSTALL_LIBDIR}")