make and travis: cross platform compile check

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

.travis.yml

@@ -24,4 +24,4 @@ script: >
   -e GOPATH=/gopath -e TRASH_CACHE=/gopath/.trashcache
   -v /etc/passwd:/etc/passwd -v /etc/sudoers:/etc/sudoers -v /etc/sudoers.d:/etc/sudoers.d
   -v /var/run:/var/run:z -v $HOME/gopath:/gopath:Z
-  -w /gopath/src/github.com/containers/image image-test bash -c "PATH=$PATH:/gopath/bin make tools .gitvalidation validate test test-skopeo SUDO=sudo BUILDTAGS=\"$BUILDTAGS\""
+  -w /gopath/src/github.com/containers/image image-test bash -c "PATH=$PATH:/gopath/bin make cross tools .gitvalidation validate test test-skopeo SUDO=sudo BUILDTAGS=\"$BUILDTAGS\""

Makefile

@@ -6,10 +6,18 @@ SKOPEO_BRANCH = master
 # Set SUDO=sudo to run container integration tests using sudo.
 SUDO =
 
+# when cross compiling _for_ a Darwin or windows host, then we must use openpgp
+BUILD_TAGS_WINDOWS_CROSS = containers_image_ostree_stub containers_image_openpgp
+BUILD_TAGS_DARWIN_CROSS = containers_image_ostree_stub containers_image_openpgp
+# when compiling _on_ a Darwin host, then we can link against gpgme
+BUILD_TAGS_DARWIN_NATIVE = containers_image_ostree_stub
+
 ifeq ($(shell uname),Darwin)
-DARWIN_BUILD_TAG = containers_image_ostree_stub
+PLATFORM_BUILD_TAG = $(BUILD_TAGS_DARWIN_NATIVE)
 endif
-BUILDTAGS = btrfs_noversion libdm_no_deferred_remove $(DARWIN_BUILD_TAG)
+
+
+BUILDTAGS = btrfs_noversion libdm_no_deferred_remove $(PLATFORM_BUILD_TAG)
 BUILDFLAGS := -tags "$(BUILDTAGS)"
 
 PACKAGES := $(shell go list $(BUILDFLAGS) ./... | grep -v github.com/containers/image/vendor)
@@ -25,8 +33,14 @@ GPGME_ENV = CGO_CFLAGS="$(shell gpgme-config --cflags 2>/dev/null)" CGO_LDFLAGS=
 
 all: tools test validate .gitvalidation
 
-build: vendor
-	@$(GPGME_ENV) go build $(BUILDFLAGS) $(PACKAGES)
+build: vendor build-internal
+
+build-internal:
+	$(GPGME_ENV) go build $(BUILDFLAGS) $(PACKAGES)
+
+cross: vendor
+	GOOS=windows $(MAKE) build-internal BUILDTAGS="$(BUILDTAGS) $(BUILD_TAGS_WINDOWS_CROSS)"
+	GOOS=darwin $(MAKE) build-internal BUILDTAGS="$(BUILDTAGS) $(BUILD_TAGS_DARWIN_CROSS)"
 
 tools: tools.timestamp