ignore-value: handle pointer types, too

ignore-value: handle pointer types, too

* lib/ignore-value.h (__attribute__): Remove definition.
(ignore_value): Remove use of "__attribute__ ((unused))" in favor
of a more concise and more-often effective "(void) i" statement.
(ignore_ptr): New function to suppress warnings from functions that
return pointers, and to make it explicit that one function doesn't
handle all cases.