make: more aggressively ensure static linkage

`ldd` returns the same thing if something is statically linked as it
does when it is dynamically linked to a library `ldd` can't find.  Thus,
the return value alone is not a reliable indicator of whether or not
something is statically linked.

In addition, check that `ldd` actually prints to stdout the expected
string indicating the binary is not dynamically linked.

Makefile

@@ -891,7 +891,7 @@ $(BUILD)/userland.tar: \
 	rm -f $(SLASHBR)/strata/local
 	# ensure static
 	for bin in $(SLASHBR)/bin/* $(SLASHBR)/libexec/*; do \
-		if ldd "$$bin" >/dev/null 2>&1; then \
+		if ldd "$$bin" >/dev/null 2>&1 || ! ldd "$$bin" 2>&1 | grep -q "not a dynamic executable" ; then \
 			echo "error: $$bin is dynamically linked"; exit 1; \
 		fi; \
 	done