Added PREFIX variable (def:/usr/local) for install

Instead of hard coding without any option to override the installation
prefix directory, I added a PREFIX variable with the default value of
/usr/local.

Also used $(APP) for binary basename references instead of hardcoding
"pit".  This way if you want to have two versions of pit running you can
rename one to, say, pit2.

Makefile

@@ -1,5 +1,7 @@
 #!/usr/bin/make
-#
+
+# Prefix directory where pit binary gets installed to.
+PREFIX = /usr/local
 
 # The toplevel directory of the source tree.
 SRCDIR = ./src
@@ -84,7 +86,7 @@ $(OBJDIR)/util.o: $(SRCDIR)/util.c $(SRCDIR)/pit.h $(SRCDIR)/object.h $(SRCDIR)/
 	$(CC) -o $(OBJDIR)/util.o -c $(SRCDIR)/util.c
 
 install:
-	cp $(BINDIR)/pit /usr/local/bin
+	cp $(BINDIR)/$(APP) $(PREFIX)/bin/$(APP)
 
 clean:
 	rm -f $(OBJDIR)/*.o