Make Makefile more compliant with conventions

CC is now assigned with `?=` to gcc, added install target,
PREFIX is respected in install target
and assigned with ?= to default to /usr/local/bin/.

Makefile

@@ -1,9 +1,11 @@
 .PHONY: all clean
 
-CC = gcc
+CC ?= gcc
 CLIBS = -lwayland-client
 CFLAGS = $(CLIBS)
 
+PREFIX ?= /usr/local/bin/
+
 SRC = main.c
 OUT = wl-globals
 
@@ -12,5 +14,8 @@ all: $(OUT)
 clean:
 	rm -f $(OUT)
 
+install:
+	cp $(OUT) $(PREFIX)
+
 $(OUT): $(SRC)
 	$(CC) $(SRC) $(CFLAGS) -o $(OUT)