Makefile: Do not depend on the "all" target for "install".

Instead, check if the binary was built beforehand with an if clause.
This helps avoid rebuilding the project as root by accident.

Makefile

@@ -59,7 +59,16 @@ test-tx:
 clean:
 	rm -f $(BINS)
 
-install: all
+install:
+	@for i in $(BINS); \
+	do \
+		if test ! -f $$i; \
+		then \
+			echo "The '$$i' binary was not built."; \
+			echo "You should run 'make BINS=$$i' as a normal user before installing."; \
+			exit 1; \
+		fi; \
+	done;
 	mkdir -p $(DESTDIR)$(PREFIX)/bin
 	cp -f $(BINS) $(DESTDIR)$(PREFIX)/bin