Update makefile

That's what we usually do.

makefile

@@ -4,7 +4,7 @@ LDFLAGS=-lX11 -lXext
 SOURCES=main.cpp x.cpp options.cpp
 OBJECTS=$(SOURCES:.cpp=.o)
 EXECUTABLE=slop
-BINDIR="usr/bin"
+BINDIR="/usr/bin"
 
 all: $(SOURCES) $(EXECUTABLE)
 
@@ -16,4 +16,4 @@ clean:
 
 install: all
 	mkdir -p $(DESTDIR)/$(BINDIR)
-	cp $(CURDIR)/$(EXECUTABLE) $(DESTDIR)/$(BINDIR)
+	cp $(CURDIR)/$(EXECUTABLE) $(DESTDIR)$(BINDIR)