ROOT6: alien support fails silently when xrootd not selected (cmake)

Dear experts,

When trying to compile root 6.07.06 with alien support, all seemed find, but the libRAliEn.so was not built (while root-config --features showed alien support).

Looking a bit through the cmake files showed that in net/CMakeLists.txt:

if(xrootd)
add_subdirectory(netx)
if(alien)
add_subdirectory(alien)
endif()
endif()

so that alien is ignored without warning when xrootd is not confirgured. Could you add a warning in the cmake system to flag this? I guess FindALIEN.cmake should check that xrootd is enabled?

On the documentation page:

https://root.cern.ch/installing-root-source
I would suggest to add under the 'external libraries', that the following variables are needed for XROOTD and ALIEN:

-DALIEN_DIR=$ALIEN_DIR/api -DXROOTD_ROOT_DIR=$ALIEN_DIR/api

Thanks, and best regards,

Marco.