Makefile: Use a pattern rule for cross-compilation

Pattern-rule documentation is in [1].  This commit follows the basic
approach from [2], with the portable build tags from [3].

Using --keep-going allows folks to see errors for multiple target
platforms.  For example, if the Darwin target dies, we'll still
attempt to build the Linux target before erroring out.

I've added an ALLOWED_TO_FAIL environment variable to mark script
blocks for the the allow_failures block.  Currently we're requiring
builds from Linux for Linux and OS X to succeed, but allowing builds
from OS X to both targets to fail.

[1]: https://www.gnu.org/software/make/manual/html_node/Pattern-Intro.html#Pattern-Intro
[2]: e5031fcf9a
[3]: https://github.com/kubernetes-incubator/cri-o/pull/1653

Signed-off-by: W. Trevor King <wking@tremily.us>

Closes: #1034
Approved by: baude

.travis.yml

@@ -7,10 +7,10 @@ services:
   - docker
 
 before_install:
-  - sudo apt-get -qq update
+  - if [ "${TRAVIS_OS_NAME}" = linux ]; then sudo apt-get -qq update; fi
-  - sudo apt-get -qq install btrfs-tools libdevmapper-dev libgpgme11-dev libapparmor-dev
+  - if [ "${TRAVIS_OS_NAME}" = linux ]; then sudo apt-get -qq install btrfs-tools libdevmapper-dev libgpgme11-dev libapparmor-dev; fi
-  - sudo apt-get -qq install autoconf automake bison e2fslibs-dev libfuse-dev libtool liblzma-dev gettext python3-setuptools python3-dateutil
+  - if [ "${TRAVIS_OS_NAME}" = linux ]; then sudo apt-get -qq install autoconf automake bison e2fslibs-dev libfuse-dev libtool liblzma-dev gettext python3-setuptools python3-dateutil; fi
-  - sudo make install.libseccomp.sudo
+  - if [ "${TRAVIS_OS_NAME}" = linux ]; then sudo make install.libseccomp.sudo; fi
 
 install:
   - make install.tools
@@ -37,11 +37,21 @@ jobs:
       script:
         - make testunit
       go: 1.10.x
+    - script:
+        - make --keep-going local-cross
+      go: 1.10.x
+    - script:
+        - make --keep-going local-cross
+      go: 1.10.x
+      os: osx
+      env: ALLOWED_TO_FAIL=true
     - stage: Integration Test
       script:
         - make all
         - make integration
       go: 1.9.x
+  allow_failures:
+    - env: ALLOWED_TO_FAIL=true
 
 notifications:
   irc: "chat.freenode.net#podman"

Makefile

@@ -18,7 +18,7 @@ ETCDIR_LIBPOD ?= ${ETCDIR}/crio
 TMPFILESDIR ?= ${PREFIX}/lib/tmpfiles.d
 SYSTEMDDIR ?= ${PREFIX}/lib/systemd/system
 BUILDTAGS ?= seccomp $(shell hack/btrfs_tag.sh) $(shell hack/libdm_tag.sh) $(shell hack/btrfs_installed_tag.sh) $(shell hack/ostree_tag.sh) $(shell hack/selinux_tag.sh) varlink
-BUILDTAGS_DARWIN ?= containers_image_ostree_stub containers_image_openpgp
+BUILDTAGS_CROSS ?= containers_image_openpgp containers_image_ostree_stub exclude_graphdriver_btrfs exclude_graphdriver_devicemapper exclude_graphdriver_overlay
 ifneq (,$(findstring varlink,$(BUILDTAGS)))
 	PODMAN_VARLINK_DEPENDENCIES = cmd/podman/varlink/ioprojectatomicpodman.go
 endif
@@ -57,6 +57,10 @@ GOMD2MAN ?= $(shell command -v go-md2man || echo '$(GOBIN)/go-md2man')
 
 BOX="fedora_atomic"
 
+CROSS_BUILD_TARGETS := \
+	bin/podman.cross.darwin.amd64 \
+	bin/podman.cross.linux.amd64
+
 all: binaries docs
 
 default: help
@@ -98,8 +102,13 @@ test/checkseccomp/checkseccomp: .gopathok $(wildcard test/checkseccomp/*.go)
 podman: .gopathok $(PODMAN_VARLINK_DEPENDENCIES)
 	$(GO) build -i -ldflags '$(LDFLAGS_PODMAN)' -tags "$(BUILDTAGS)" -o bin/$@ $(PROJECT)/cmd/podman
 
-darwin:
+local-cross: $(CROSS_BUILD_TARGETS)
-	GOOS=darwin $(GO) build -ldflags '$(LDFLAGS_PODMAN)' -tags "$(BUILDTAGS_DARWIN)" -o bin/podman.$@ $(PROJECT)/cmd/podman
+
+bin/podman.cross.%: .gopathok
+	TARGET="$*"; \
+	GOOS="$${TARGET%%.*}" \
+	GOARCH="$${TARGET##*.}" \
+	$(GO) build -i -ldflags '$(LDFLAGS_PODMAN)' -tags '$(BUILDTAGS_CROSS)' -o "$@" $(PROJECT)/cmd/podman
 
 python-podman:
 ifdef HAS_PYTHON3
@@ -110,7 +119,7 @@ clean:
 	rm -rf \
 		.gopathok \
 		_output \
-		bin/podman \
+		bin \
 		build \
 		test/bin2img/bin2img \
 		test/checkseccomp/checkseccomp \