Fixup a duplicate slash, make install target depend on all

PREFIX should be the containing dir for program files, /bin/ should
be appended by the target itself. Additionally, the install target now
creates the target directory if it doesn't exist.

Makefile

@@ -4,7 +4,7 @@ CC ?= gcc
 CLIBS = -lwayland-client
 CFLAGS = $(CLIBS)
 
-PREFIX ?= /usr/local/bin/
+PREFIX ?= /usr/local
 
 SRC = main.c
 OUT = wl-globals
@@ -14,8 +14,9 @@ all: $(OUT)
 clean:
 	rm -f $(OUT)
 
-install:
-	cp $(OUT) $(PREFIX)
+install: all
+	mkdir -p "$(PREFIX)/bin/"
+	cp $(OUT) "$(PREFIX)/bin/"
 
 $(OUT): $(SRC)
 	$(CC) $(SRC) $(CFLAGS) -o $(OUT)