scripts/build/Makefile: fix

Unify alpine and non-alpine builds. The only difference is foreign
arch builds need some preparation -- separate that to a dependency.

Unfortunately we can't use wildcard targets ("%: ") as non-wildcard
ones are prevaling. Therefore, a somewhat ugly hack to generate
$arch: Dockerfile.$arch dependency is added.

While at it:
 - rename DB_ARGS to DB_CC
 - mark clean as phony
 - make "all" really run all the builds we can

Nice side effect: autocompletion ("make -C scripts/build <TAB>")
now works!
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

scripts/build/Makefile

-ARCHES ?= armv7hf aarch64 ppc64le # x86_64
-all: $(ARCHES)
+QEMU_ARCHES := armv7hf aarch64 ppc64le # require qemu
+ARCHES := $(QEMU_ARCHES) x86_64
+TARGETS := $(ARCHES) alpine
+TARGETS_CLANG := $(addsuffix $(TARGETS),-clang)
+
+all: $(TARGETS) $(TARGETS_CLANG)
+.PHONY: all
+
+# A build for each architecture requires appropriate Dockerfile
+define ARCH_DEP
+$(1): Dockerfile.$(1)
+endef
+$(foreach arch,$(ARCHES),$(eval $(call ARCH_DEP,$(arch))))
 
 Dockerfile.%: Dockerfile.%.hdr Dockerfile.tmpl
 	cat $^ > $@
@@ -11,21 +22,22 @@ binfmt_misc:
 	./binfmt_misc
 .PHONY: binfmt_misc
 
-alpine:
-	docker build  -t criu-$@ -f Dockerfile.alpine $(DB_ARGS) ../..
+$(QEMU_ARCHES): qemu-user-static binfmt_misc
+
+$(TARGETS):
+	docker build  -t criu-$@ -f Dockerfile.$@ $(DB_CC) ../..
+.PHONY: $(TARGETS)
 
-%: Dockerfile.% qemu-user-static binfmt_misc
-	docker build  -t criu-$@ -f $< $(DB_ARGS) ../..
+# Clang builds add some Docker build env
+define CLANG_DEP
+$(1)-clang: $(1)
+endef
+$(foreach t,$(TARGETS),$(eval $(call CLANG_DEP,$(t))))
 
-%-clang: DB_ARGS=--build-arg CC=clang-3.8
-alpine-clang: DB_ARGS=--build-arg CC=clang
-alpine-clang: alpine
-armv7hf-clang: armv7hf
-aarch64-clang: DB_ARGS=--build-arg CC=clang-3.6
-aarch64-clang: aarch64
-ppc64le-clang: ppc64le
-x86_64-clang: x86_64
-.PHONY: alpine-clang armv7hf-clang aarch64-clang ppc64le-clang x86_64-clang
+%-clang:	DB_CC=--build-arg CC=clang-3.8
+alpine-clang:	DB_CC=--build-arg CC=clang
+aarch64-clang:	DB_CC=--build-arg CC=clang-3.6
+.PHONY: $(TARGETS_CLANG)
 
 clean:
 	rm -rf qemu-user-static
@@ -34,3 +46,4 @@ clean:
 		test -f $$FILE && echo -1 > $$FILE; \
 		rm -f Dockerfile.$$ARCH; \
 	done
+.PHONY: clean