Improve Makefile

 - Installation directory overridable via PREFIX arg, e.g. `make
   PREFIX=/somewhere install`

 - `compile` target removed in preference of real dependencies,
   i.e. `realpath` file depends on `realpath.c`

 - `install` target added to `.PHONY`

Makefile

@@ -1,6 +1,10 @@
-BIN=realpath
-compile:
-	gcc -Wall -o ${BIN} ${BIN}.c
-install:
-	cp -f ${BIN} /usr/bin/${BIN}
+PREFIX=/usr/local/bin
+PROG=realpath
+
+.PHONY: install
 
+${PROG}: ${PROG}.c
+	gcc -Wall -o $@ $^
+
+install:
+	cp -f ${PROG} ${PREFIX}/${PROG}