Fix Makefile for Windows builds (#566)

Fixes #564 (/gitea/tea/issues/564) by applying minor changes to the Makefile.
- Defines a `BINEXT` variable which holds a file name extension (.exe for Windows, empty otherwise)
- Replaces uses of `$(EXECUTABLE)` with `tea$(EXECUTABLE)`
- appends `$(BINEXT)` to the custom vet tool flag

Besides that I made two minor changes
- Add `[.exe]` to the ignore rule for gitea-vet, so it also ignores the binary when built on Windows hosts
- Removed `$(DIST)/binaries` from release-dirs as this is not being called anywhere else

I've tested almost all `make` tasks to work on Windows. The only one I haven't run is the docker build.

WIP tag is because I still have to test it on Linux, will do so in a bit. I also asked for some discussion on the original issue, which I'd like to do before blindly asking to merge in changes

Reviewed-on: https://gitea.com/gitea/tea/pulls/566
Co-authored-by: Martin Veldwijk <riven@tae.moe>
Co-committed-by: Martin Veldwijk <riven@tae.moe>

.gitignore

@@ -1,5 +1,5 @@
 tea
-/gitea-vet
+/gitea-vet[.exe]
 
 .idea/
 .history/

Makefile

@@ -36,8 +36,10 @@ SOURCES ?= $(shell find . -name "*.go" -type f)
 # OS specific vars.
 ifeq ($(OS), Windows_NT)
 	EXECUTABLE := tea.exe
+	VET_TOOL := gitea-vet.exe
 else
 	EXECUTABLE := tea
+	VET_TOOL := gitea-vet
 	ifneq ($(shell uname -s), OpenBSD)
 		override BUILDMODE := -buildmode=pie
 	endif
@@ -61,7 +63,7 @@ vet:
 	$(GO) vet $(PACKAGES)
 	# Custom vet
 	$(GO) build code.gitea.io/gitea-vet
-	$(GO) vet -vettool=gitea-vet $(PACKAGES)
+	$(GO) vet -vettool=$(VET_TOOL) $(PACKAGES)
 
 .PHONY: lint
 lint: install-lint-tools
@@ -120,7 +122,7 @@ release: release-dirs install-release-tools release-os release-compress release-
 
 .PHONY: release-dirs
 release-dirs:
-	mkdir -p $(DIST)/binaries $(DIST)/release
+	mkdir -p $(DIST)/release
 
 .PHONY: release-os
 release-os: