lib: improve Makefile

Make object files explicit and add "clean" target.

Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>

src/lib/Makefile

@@ -1,7 +1,12 @@
+obj := library.o main.o
+
 all: xlib
 
+clean:
+	rm -rf xlib $(obj)
+
+xlib: $(obj)
+	gcc -Wall -o $@ $^
+
 .c.o:
 	gcc -Wall -I. -I.. -Iinclude -g -c -o $@ $<
-
-xlib: $(patsubst %.c,%.o,$(wildcard *.c))
-	gcc -Wall -o $@ $^