[CMake] Howto unset cache variable without UNSET()

[Marcel Loose]

On Wed, 2010-06-30 at 17:51 +0200, Alexander Neundorf wrote:
> On Wednesday 30 June 2010, Marcel Loose wrote:
> > Hi all,
> >
> > Is there a way to unset a cache variable, i.e. make it undefined,
> > *without* using unset().
> >
> > My reason for asking is that my CMake scripts need to be backward
> > compatible with every 2.6 version. Unfortunately, unset() was added
in a
> > patch release (2.6.3 if I recall correctly), so I cannot use it.
> >
> > The problem is that, though 'set(FOO)' will make FOO undefined, this
> > doesn't seem to work for 'set(FOO CACHE INTERNAL "Foo")'.
> 
> Does it really have to be unset or would empty or "FALSE" also do ?
> Before 2.6.3 there is no way to get something removed from the cache.
> 
> Alex

I think I could live with using an empty string and then use 
'if(VAR MATCHES "^$")', instead of 'if(DEFINED VAR)'. I was just
wondering whether I overlooked another way of doing this. Also because
setting a non-cached variable to nothing (i.e. set(VAR)), does make VAR
undefined.

Thanks,
Marcel Loose.