go1.15 on windows (#12589) (#12593)

We don't support go1.15 on 1.12.x branch, however this will allow users who chose to build with go1.15 on windows to be successful

Makefile

@@ -40,8 +40,10 @@ endif
 
 
 ifeq ($(OS), Windows_NT)
+	GOFLAGS := -v -buildmode=exe
 	EXECUTABLE ?= gitea.exe
 else
+	GOFLAGS := -v
 	EXECUTABLE ?= gitea
 	UNAME_S := $(shell uname -s)
 	ifeq ($(UNAME_S),Darwin)
@@ -54,7 +56,6 @@ endif
 
 GOFMT ?= gofmt -s
 
-GOFLAGS := -v
 EXTRA_GOFLAGS ?=
 
 MAKE_VERSION := $(shell $(MAKE) -v | head -n 1)
@@ -531,7 +532,7 @@ release-windows: | $(DIST_DIRS)
 	@hash xgo > /dev/null 2>&1; if [ $$? -ne 0 ]; then \
 		GO111MODULE=off $(GO) get -u src.techknowlogick.com/xgo; \
 	fi
-	CGO_CFLAGS="$(CGO_CFLAGS)" GO111MODULE=off xgo -go $(XGO_VERSION) -dest $(DIST)/binaries -tags 'netgo osusergo $(TAGS)' -ldflags '-linkmode external -extldflags "-static" $(LDFLAGS)' -targets 'windows/*' -out gitea-$(VERSION) .
+	CGO_CFLAGS="$(CGO_CFLAGS)" GO111MODULE=off xgo -go $(XGO_VERSION) -buildmode exe -dest $(DIST)/binaries -tags 'netgo osusergo $(TAGS)' -ldflags '-linkmode external -extldflags "-static" $(LDFLAGS)' -targets 'windows/*' -out gitea-$(VERSION) .
 ifeq ($(CI),drone)
 	cp /build/* $(DIST)/binaries
 endif