Do CI compile with xgo

This will hopefully fix Travis errors:

/home/travis/.gimme/versions/go1.11.7.linux.amd64/pkg/tool/linux_amd64/link: running gcc failed: exit status 1
/usr/bin/ld: /tmp/go-link-810454258/000020.o: unrecognized relocation (0x2a) in section `.text'
/usr/bin/ld: final link failed: Bad value

Makefile

@@ -13,8 +13,8 @@ TMPBIN=./tmp
 
 all : build
 
-ci: ci-assets deps
+ci: ci-assets deps $(TMPBIN)/xgo
-	cd cmd/writefreely; $(GOBUILD) -v -tags='sqlite'
+	$(TMPBIN)/xgo -v -tags='sqlite' ./cmd/writefreely
 
 build: assets deps
 	cd cmd/writefreely; $(GOBUILD) -v -tags='sqlite'
@@ -117,6 +117,9 @@ $(TMPBIN):
 $(TMPBIN)/go-bindata: deps $(TMPBIN)
 	$(GOBUILD) -o $(TMPBIN)/go-bindata github.com/jteeuwen/go-bindata/go-bindata
 
+$(TMPBIN)/xgo: deps $(TMPBIN)
+	$(GOBUILD) -o $(TMPBIN)/xgo github.com/karalabe/xgo
+
 ci-assets : $(TMPBIN)/go-bindata
 	$(TMPBIN)/go-bindata -pkg writefreely -ignore=\\.gitignore schema.sql sqlite.sql