Fixing make file so you can build linx on mac easily (#4058)

Makefile

@@ -42,6 +42,9 @@ GO_LINKER_FLAGS ?= -ldflags \
 				    -X github.com/mattermost/platform/model.BuildHashEnterprise=$(BUILD_HASH_ENTERPRISE)\
 				    -X github.com/mattermost/platform/model.BuildEnterpriseReady=$(BUILD_ENTERPRISE_READY)"
 
+# GOOS/GOARCH of the build host, used to determine whether we're cross-compiling or not
+BUILDER_GOOS_GOARCH="$(shell $(GO) env GOOS)_$(shell $(GO) env GOARCH)"
+
 # Output paths
 DIST_ROOT=dist
 DIST_PATH=$(DIST_ROOT)/mattermost
@@ -310,7 +313,11 @@ endif
 
 	@# Make osx package
 	@# Copy binary
-	cp $(GOPATH)/bin/darwin_amd64/platform $(DIST_PATH)/bin
+ifeq ($(BUILDER_GOOS_GOARCH),"darwin_amd64")
+	cp $(GOPATH)/bin/platform $(DIST_PATH)/bin # from native bin dir, not cross-compiled
+else
+	cp $(GOPATH)/bin/darwin_amd64/platform $(DIST_PATH)/bin # from cross-compiled bin dir
+endif
 	@# Package
 	tar -C dist -czf $(DIST_PATH)-$(BUILD_TYPE_NAME)-osx-amd64.tar.gz mattermost
 	@# Cleanup
@@ -318,7 +325,11 @@ endif
 
 	@# Make windows package
 	@# Copy binary
-	cp $(GOPATH)/bin/windows_amd64/platform.exe $(DIST_PATH)/bin
+ifeq ($(BUILDER_GOOS_GOARCH),"windows_amd64")
+	cp $(GOPATH)/bin/platform.exe $(DIST_PATH)/bin # from native bin dir, not cross-compiled
+else
+	cp $(GOPATH)/bin/windows_amd64/platform.exe $(DIST_PATH)/bin # from cross-compiled bin dir
+endif
 	@# Package
 	tar -C dist -czf $(DIST_PATH)-$(BUILD_TYPE_NAME)-windows-amd64.tar.gz mattermost
 	@# Cleanup
@@ -326,10 +337,14 @@ endif
 
 	@# Make linux package
 	@# Copy binary
-	cp $(GOPATH)/bin/platform $(DIST_PATH)/bin
+ifeq ($(BUILDER_GOOS_GOARCH),"linux_amd64")
+	cp $(GOPATH)/bin/platform $(DIST_PATH)/bin # from native bin dir, not cross-compiled
+else
+	cp $(GOPATH)/bin/linux_amd64/platform $(DIST_PATH)/bin # from cross-compiled bin dir
+endif
 	@# Package
 	tar -C dist -czf $(DIST_PATH)-$(BUILD_TYPE_NAME)-linux-amd64.tar.gz mattermost
-	@# Don't cleanup linux package so dev machines will have an unziped linux package avalilable
+	@# Don't clean up native package so dev machines will have an unzipped package available
 	@#rm -f $(DIST_PATH)/bin/platform