The `CheckZlib.cmake` module uses `NULL` in the test source code to
check for the `zError()` symbol. This fails to build on FreeBSD 10.3
with the following error:
CheckPrototypeDefinition.c:15:10: error: use of undeclared identifier 'NULL'
return NULL;
^
I don't know the root cause for this failure, but including `stddef.h`
in this test source code fixes the problem.
PR: 225501
Reported by: cpm@
8936ec8373