ClamD's STATS API reports process memory stats on systems that
provide the `mallinfo()` system call.
This feature is used by ClamDTOP to show process memory usage.
When we switched to the CMake build system, we neglected to add the
check for the `mallinfo()` system call and so broke ClamD memory
usage reporting.
This commit adds the CMake check for `mallinfo()` and sets
HAVE_MALLINFO, if found.
Fixes: https://github.com/Cisco-Talos/clamav/issues/706
Jira: CLAM-2742
