Properly handle rpath on macOS

Without this, the install_name of JAlienROOT is libJAlienROOT rather than
@rpath/libJAlienROOT, with the result things might break when SIP is
enabled.

CMakeLists.txt

 cmake_minimum_required(VERSION 2.8.12 FATAL_ERROR)
 project(JAliEn-ROOT CXX)
+
+cmake_policy(SET CMP0068 NEW)
+
 list(APPEND CMAKE_PREFIX_PATH ${ROOTSYS})
 set(CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/cmake/modules")
 find_package(JSONC REQUIRED)
@@ -9,6 +12,29 @@ find_package(ROOT REQUIRED COMPONENTS Net NetxNG Tree XMLIO)
 find_package(ZLIB REQUIRED)
 find_package(XROOTD REQUIRED)
 
+if(APPLE)
+  set(CMAKE_MACOSX_RPATH ON)
+  set(basePoint @loader_path)
+else()
+  set(basePoint $ORIGIN)
+endif()
+
+# use, i.e. do not skip, the full RPATH in the _build_ tree
+set(CMAKE_SKIP_BUILD_RPATH FALSE)
+# when building, do not use the install RPATH already (will only be used when
+# actually installing), unless we are on a Mac (where the install is otherwise
+# pretty slow)
+set(CMAKE_BUILD_WITH_INSTALL_RPATH FALSE)
+if(APPLE)
+  set(CMAKE_BUILD_WITH_INSTALL_RPATH TRUE)
+endif()
+
+# add to the install RPATH the (automatically determined) parts of the RPATH
+# that point to directories outside the build tree
+set(CMAKE_INSTALL_RPATH_USE_LINK_PATH TRUE)
+
+set(CMAKE_INSTALL_RPATH ${basePoint} ${basePoint}/../lib)
+
 foreach(FEATURE xrootd xml)
   if(NOT ROOT_${FEATURE}_FOUND)
     message(FATAL_ERROR "ROOT was built without ${FEATURE} support (required)")
@@ -70,6 +96,7 @@ set(HDRS
     TAliceJobStatus.h
     )
 
+
 ROOT_GENERATE_DICTIONARY(G__${LIBNAME} "${HDRS}" LINKDEF LinkDef.h OPTIONS -Iinc)
 add_library(${LIBNAME} SHARED "${SRCS}" G__${LIBNAME}.cxx)
 target_link_libraries(${LIBNAME} ${ROOT_LIBRARIES} json-c websockets ${OPENSSL_LIBRARIES} ${ZLIB_LIBRARIES} ${XROOTD_LIBRARIES})