link: improve Makefile

Make object filenames explicit and add "clean" target.

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

src/link/Makefile

@@ -1,8 +1,21 @@
+obj := \
+	assign.o \
+	library.o \
+	main.o \
+	mapfile.o \
+	object.o \
+	output.o \
+	patch.o \
+	symbol.o
+
 all: xlink
 
+clean:
+	rm -rf xlink $(obj)
+
+xlink: $(obj)
+	gcc -Wall -o $@ $^
+
 .c.o:
 	gcc -Wall -I. -I.. -Iinclude -g -c -o $@ $<
 
-xlink: $(patsubst %.c,%.o,$(wildcard *.c))
-	gcc -Wall -o $@ $^
-