NetBSD needs -D_OPENBSD_SOURCE in order to use reallocarray(3)

diff --git a/Makefile.am b/Makefile.am
index 81dc4c7..1685c3d 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -63,6 +63,11 @@ if IS_AIX
 DEFS += -D_LINUX_SOURCE_COMPAT=1
 endif
 
+# Set flags for NetBSD.
+if IS_NETBSD
+CPPFLAGS += -D_OPENBSD_SOURCE
+endif
+
 # List of sources.
 dist_tmux_SOURCES = \
 	alerts.c \