complibs/cloog: add support for the ISL backend

CLooG 0.18+ will use ISL instead of PPL, so we have to configure
adequately depending of which backend is in use.

The Kconfig entries will decide for us which is selected, so we
can rely on either PPL xor ISL to be selected, not both.

Reported-by: "Plotnikov Dmitry" <leitz@ispras.ru>
[Dmitry did a preliminray patch to add ISL support,
which this patch is inspired from]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

 # dmalloc debug facility

 ## depends on ! BACKEND

 ##

 ## help dmalloc helps tracking memory allocation mis-usage

 ## help such as double-free, use-after-free...

 choice

     bool

     prompt "dmalloc version"

 # Don't remove next line

 # CT_INSERT_VERSION_BELOW

 config DMALLOC_V_5_5_2

     bool

     prompt "5.5.2"

 endchoice

 config DMALLOC_VERSION

     string

 # Don't remove next line

 # CT_INSERT_VERSION_STRING_BELOW

     default "5.5.2" if DMALLOC_V_5_5_2