complibs/cloog: fix linking with libstdc++

This rules out 0.15.5 and previous versions, that did not
have this option, so remove them from the list. Anyway,
they were marked 'OBSOLETE', so it's not a big loss...

[Yann E. MORIN: remove obsolete versions]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # Compute Alpha-specific values

 CT_DoArchTupleValues () {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}${CT_ARCH_ALPHA_VARIANT}"

 }