Update build to cross compile

Currently targeting linux/amd64 and darwin/amd64

.gitignore

@@ -4,8 +4,10 @@
 
 # build artifacts
 coverage
-opa
 ast/parser.go
+opa
+opa_linux_amd64
+opa_darwin_amd64
 
 # runtime artifacts
 policies

.travis.yml

@@ -1,4 +1,6 @@
 language: go
+env:
+  - CROSSCOMPILE="linux/amd64 darwin/amd64"
 go:
   - 1.5
   - 1.6

Makefile

@@ -10,6 +10,9 @@ PACKAGES := \
 	github.com/open-policy-agent/opa/storage/.../ \
 	github.com/open-policy-agent/opa/util/.../
 
+GO := go
+GOX := gox
+
 BUILD_COMMIT := $(shell ./build/get-build-commit.sh)
 BUILD_TIMESTAMP := $(shell ./build/get-build-timestamp.sh)
 BUILD_HOSTNAME := $(shell ./build/get-build-hostname.sh)
@@ -18,7 +21,9 @@ LDFLAGS := -ldflags "-X github.com/open-policy-agent/opa/version.Vcs=$(BUILD_COM
 	-X github.com/open-policy-agent/opa/version.Timestamp=$(BUILD_TIMESTAMP) \
 	-X github.com/open-policy-agent/opa/version.Hostname=$(BUILD_HOSTNAME)"
 
-GO := go
+# Set CROSSCOMPILE to space separated list of <platform>/<arch> pairs
+# and "gox" will be used to build the binaries instead of "go".
+CROSSCOMPILE ?=
 
 GO15VENDOREXPERIMENT := 1
 export GO15VENDOREXPERIMENT
@@ -31,12 +36,17 @@ deps:
 	$(GO) install ./vendor/github.com/PuerkitoBio/pigeon
 	$(GO) install ./vendor/golang.org/x/tools/cmd/goimports
 	$(GO) install ./vendor/github.com/golang/lint/golint
+	$(GO) get github.com/mitchellh/gox
 
 generate:
 	$(GO) generate
 
 build: generate
+ifeq ($(CROSSCOMPILE),)
 	$(GO) build -o opa $(LDFLAGS)
+else
+	$(GOX) -osarch="$(CROSSCOMPILE)" $(LDFLAGS)
+endif
 
 install: generate
 	$(GO) install $(LDFLAGS)
@@ -68,3 +78,5 @@ fmt:
 
 clean:
 	rm -f ./opa
+	rm -f ./opa_linux_amd64
+	rm -f ./opa_darwin_amd64

docs/dev/RELEASE.md

@@ -68,7 +68,7 @@ CHANGELOG.md snippet and uploading the packages from the build phase.
 1. Run command to build packages. This will produce a bunch of binaries (e.g., amd64/linux, i386/linux, amd64/darwin, etc.) that can be published (“distributions”).
 
 	```
-	make dist
+	make build CROSSCOMPILE="linux/amd64 darwin/amd64"
 	```
 
 ## Publishing