MXS-2366: Fix RPATH setting for tarball

All RPATH setting have to be set before generation of any binary.
Defining of CMAKE_INSTALL_RPATH is moved to separate file which is called from main CMakeLists.txt in the beginning, just after install_layout.cmake

cmake/rpath.cmake

@@ -0,0 +1,8 @@
+if(PACKAGE AND TARBALL)
+  set(CMAKE_INSTALL_RPATH_USE_LINK_PATH TRUE)
+  set(CMAKE_INSTALL_RPATH "$ORIGIN/../${CMAKE_INSTALL_LIBDIR}/maxscale" CACHE PATH "" FORCE)
+else()
+  set(CMAKE_INSTALL_RPATH ${CMAKE_INSTALL_RPATH}:${CMAKE_INSTALL_PREFIX}/${MAXSCALE_LIBDIR})
+endif()
+
+