Makefile: add STATIC=true for static PIE builds (#349)

- `make build` + `make install` now support the `STATIC=true` parameter, creating statically linked builds that are also position independent executables
- this requires CGO and a static libc on the build system
- `CGO_ENABLED=0` is set for all make build targets, unless `STATIC=true` is set
- Debug symbols are stripped (`-s -w`) for all make build targets
- Release binaries are built statically by gox (no PIE), as before.

I also took the liberty to declutter the makefile from unused & duplicated variables.

Co-authored-by: Norwin Roosen <git@nroo.de>
Co-authored-by: 6543 <6543@obermui.de>
Reviewed-on: https://gitea.com/gitea/tea/pulls/349
Reviewed-by: Andrew Thornton <art27@cantab.net>
Reviewed-by: 6543 <6543@obermui.de>
Co-authored-by: Norwin <noerw@noreply.gitea.io>
Co-committed-by: Norwin <noerw@noreply.gitea.io>

Dockerfile

@@ -2,16 +2,16 @@ ARG GOVERSION="1.16.2"
 
 FROM golang:${GOVERSION}-alpine AS buildenv
 
-ARG CGO_ENABLED="0"
 ARG GOOS="linux"
 
 COPY . $GOPATH/src/
 WORKDIR $GOPATH/src
 
 RUN	apk add --quiet --no-cache \
+		build-base \
 		make \
 		git && \
-	make build
+	make clean build STATIC=true
 
 FROM scratch
 ARG VERSION="0.7.0"