Allow specification of general Go build flags and ldflags

This is needed if you need to customise the build config for a given platform.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

Makefile

@@ -27,18 +27,18 @@ SHELL := $(shell command -v bash 2>/dev/null)
 .DEFAULT: runc
 
 runc: $(SOURCES)
-	go build -i -ldflags "-X main.gitCommit=${COMMIT} -X main.version=${VERSION}" -tags "$(BUILDTAGS)" -o runc .
+	go build -i $(EXTRA_FLAGS) -ldflags "-X main.gitCommit=${COMMIT} -X main.version=${VERSION} $(EXTRA_LDFLAGS)" -tags "$(BUILDTAGS)" -o runc .
 
 all: runc recvtty
 
 recvtty: contrib/cmd/recvtty/recvtty
 
 contrib/cmd/recvtty/recvtty: $(SOURCES)
-	go build -i -ldflags "-X main.gitCommit=${COMMIT} -X main.version=${VERSION}" -tags "$(BUILDTAGS)" -o contrib/cmd/recvtty/recvtty ./contrib/cmd/recvtty
+	go build -i $(EXTRA_FLAGS) -ldflags "-X main.gitCommit=${COMMIT} -X main.version=${VERSION} $(EXTRA_LDFLAGS)" -tags "$(BUILDTAGS)" -o contrib/cmd/recvtty/recvtty ./contrib/cmd/recvtty
 
 static: $(SOURCES)
-	CGO_ENABLED=1 go build -i -tags "$(BUILDTAGS) cgo static_build" -ldflags "-w -extldflags -static -X main.gitCommit=${COMMIT} -X main.version=${VERSION}" -o runc .
-	CGO_ENABLED=1 go build -i -tags "$(BUILDTAGS) cgo static_build" -ldflags "-w -extldflags -static -X main.gitCommit=${COMMIT} -X main.version=${VERSION}" -o contrib/cmd/recvtty/recvtty ./contrib/cmd/recvtty
+	CGO_ENABLED=1 go build -i $(EXTRA_FLAGS) -tags "$(BUILDTAGS) cgo static_build" -ldflags "-w -extldflags -static -X main.gitCommit=${COMMIT} -X main.version=${VERSION} $(EXTRA_LDFLAGS)" -o runc .
+	CGO_ENABLED=1 go build -i $(EXTRA_FLAGS) -tags "$(BUILDTAGS) cgo static_build" -ldflags "-w -extldflags -static -X main.gitCommit=${COMMIT} -X main.version=${VERSION} $(EXTRA_LDFLAGS)" -o contrib/cmd/recvtty/recvtty ./contrib/cmd/recvtty
 
 release:
 	@flag_list=(seccomp selinux apparmor static); \
@@ -62,7 +62,7 @@ release:
 			CGO_ENABLED=1; \
 		}; \
 		echo "Building target: $$output"; \
-		go build -i -ldflags "$$ldflags" -tags "$$tags" -o "$$output" .; \
+		go build -i $(EXTRA_FLAGS) -ldflags "$$ldflags $(EXTRA_LDFLAGS)" -tags "$$tags" -o "$$output" .; \
 	done
 
 dbuild: runcimage