[MacOS] Change CMake installed names to absolute paths from @rpath/...

gtk-mac-bundler can't access the executable's rpath list so it can't
follow dependencies if they're not in $install_dir/lib from @rpath.
Autotools always sets absolute path install names so this should have no
adverse affects on other Mac builds.

libgnucash/backend/xml/CMakeLists.txt

@@ -115,6 +115,10 @@ IF (WIN32)
 ENDIF(WIN32)
 
 
+IF (APPLE)
+  SET_TARGET_PROPERTIES (gncmod-backend-xml PROPERTIES INSTALL_NAME_DIR "${CMAKE_INSTALL_PREFIX}/${LIB_DIR}")
+ENDIF()
+
 INSTALL(TARGETS gncmod-backend-xml
   LIBRARY DESTINATION ${LIB_DIR}
   ARCHIVE DESTINATION ${LIB_DIR}