* Makefile: add DESTDIR support

Needed mostly for distro vendors (.spec writers etc.)

* Makefile: do not strip when installing
Otherwise distros could not produce debuginfo packages

* Makefile: add BINDIR
Signed-off-by: Kir Kolyshkin <kir@openvz.org>

Makefile

 CFLAGS=-std=c99 -g -Wall -Wextra -pedantic -D_GNU_SOURCE -D_FILE_OFFSET_BITS=64
 LDFLAGS=
 PREFIX=/usr/local
+BINDIR=$(PREFIX)/bin
 
 all: ioping
 
@@ -8,7 +9,8 @@ clean:
 	$(RM) -f ioping.o ioping
 
 install: ioping
-	install -m 0755 -s ioping ${PREFIX}/bin
+	mkdir -p $(DESTDIR)$(BINDIR)
+	install -m 0755 ioping $(DESTDIR)$(BINDIR)
 
 ioping.o: ioping.c
 	$(CC) -c -o $@ $^ ${CFLAGS}