[autotools] build failure on OpenBSD/arm64 with missing refs to memset

4.19.3 builds fine on OpenBSD/amd64, but it fails to link libxfce4windowing.so on OpenBSD/arm64, apparently due to the combination of memset usage and the use of -Wl,-no-undefined.

ld: error: undefined symbol: memset
>>> referenced by xfw-wnck-icon.c:419
>>>               .libs/libxfce4windowing_0_la-xfw-wnck-icon.o:(xfw_wnck_object_get_net_wm_icon)
>>> referenced by xfw-wnck-icon.c:573
>>>               .libs/libxfce4windowing_0_la-xfw-wnck-icon.o:(xfw_wnck_object_get_wmhints_icon)
>>> referenced by xfw-wnck-icon.c:335
>>>               .libs/libxfce4windowing_0_la-xfw-wnck-icon.o:(xfw_wnck_icon_argb_to_bmp)
cc: error: linker command failed with exit code 1 (use -v to see invocation)

g_slice_new0 is a glib macro which expands to memset so it's not surprising to have 3 errors.

on some platforms/compilers, memset is a builtin, and in libc on others, but we don't link with libc.

i'll try the meson-based build system on both platforms, given that we don't know yet if autotools will still be supported for 4.20.