mem.c: check whether MADV_MERGEABLE is available

Before using MADV_MERGEABLE, check whether it's available by macro
HAVE_MADV_MERGEABLE.

Signed-off-by: DAN LI <li.dan@cn.fujitsu.com>
Reviewed-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
1 file changed