Warning about unused variables when using xalloc

[Jean-Pierre Flori]

Dear all,

It seems I messed it up when trying to do a pull request on github.
But I made a branch to suppress a lot of GCC warnings about unused parameters when building Singular in debug mode here:
* https://github.com/jpflori/singular/tree/xalloc_unused_params
It may be worth merging though I don't know to what extent it works with orther gcc versions or other compilers...

Best,
JPF