Makefile: use qcow2 tag by default if available

Signed-off-by: Koichi Shiraishi <zchee.io@gmail.com>

Author: zchee

Makefile

@@ -125,8 +125,6 @@ endif
 
 # Use mirage-block for pwritev|preadv
 HAVE_OCAML_QCOW := $(shell if ocamlfind query qcow uri >/dev/null 2>/dev/null ; then echo YES ; else echo NO; fi)
-# included 'qcow2' in the $GO_BUILD_TAGS, and $HAVE_OCAML_QCOW is YES
-ifneq (,$(findstring qcow2,$(GO_BUILD_TAGS)))
 ifeq ($(HAVE_OCAML_QCOW),YES)
 LIBEV_FILE=/usr/local/lib/libev.a
 LIBEV=$(shell if test -e $(LIBEV_FILE) ; then echo $(LIBEV_FILE) ; fi )
@@ -146,9 +144,9 @@ OCAML_LDLIBS := -L $(OCAML_WHERE) \
 	-lasmrun -lbigarray -lunix
 CGO_CFLAGS += -DHAVE_OCAML=1 -DHAVE_OCAML_QCOW=1 -DHAVE_OCAML=1 -I$(OCAML_WHERE)
 CGO_LDFLAGS += $(OCAML_LDLIBS)
+GO_BUILD_TAGS += qcow2
 bin/docker-machine-driver-xhyve: vendor/github.com/zchee/libhyperkit/mirage_block_ocaml.o
 endif
-endif
 
 
 GO_BUILD_FLAG += -tags='$(GO_BUILD_TAGS)'