cmake: Fix library ABI version on Linux

This change emulates Libtool to make sure Libtool and CMake agree on the
ABI version.

Author: hebasto

src/CMakeLists.txt

@@ -34,9 +34,13 @@ set_target_properties(secp256k1_precomputed PROPERTIES POSITION_INDEPENDENT_CODE
 target_include_directories(secp256k1 PUBLIC
   $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
 )
+
+# This emulates Libtool to make sure Libtool and CMake agree on the ABI version,
+# see below "Calculate the version variables" in build-aux/ltmain.sh.
+math(EXPR ${PROJECT_NAME}_soversion "${${PROJECT_NAME}_LIB_VERSION_CURRENT} - ${${PROJECT_NAME}_LIB_VERSION_AGE}")
 set_target_properties(secp256k1 PROPERTIES
-  VERSION "${${PROJECT_NAME}_LIB_VERSION_CURRENT}.${${PROJECT_NAME}_LIB_VERSION_AGE}.${${PROJECT_NAME}_LIB_VERSION_REVISION}"
-  SOVERSION "${${PROJECT_NAME}_LIB_VERSION_CURRENT}"
+  VERSION ${${PROJECT_NAME}_soversion}.${${PROJECT_NAME}_LIB_VERSION_AGE}.${${PROJECT_NAME}_LIB_VERSION_REVISION}
+  SOVERSION ${${PROJECT_NAME}_soversion}
 )
 
 if(SECP256K1_BUILD_BENCHMARK)