fix(build): instruct users to use GOTAGS, not GOFLAGS

[Stebalien]

GOFLAGS will override all the flags. If we just want to set a tag e.g., enable OpenSSL, we should set GOTAGS=openssl.

fixes #6622 (comment)

[olizilla]

I wasn't clear on the source of the issue, so I did some digging... and it seems some make lore has been invoked...

If a variable has been set with a command argument (see Overriding Variables), then ordinary assignments in the makefile are ignored.
https://www.gnu.org/software/make/manual/html_node/Override-Directive.html#Override-Directive

so when we

$ make -n build GOFLAGS=-tags=openssl

...The append has no affect, as GOFLAGS was already defined.

https://github.com/ipfs/go-ipfs/blob/37884530221654fe57870db31ec7a178963b633e/mk/golang.mk#L9-L14

so the use of += here is misleading, as you pointed out.

To save us from future misreadings, either GOFLAGS should append, in which case we want to specify override, or we should assign the default value for GOFLAGS as part of the ?= statement...

Appending with override

# Try to make building as reproducible as possible by stripping the go path.
override GOFLAGS += "-asmflags=all='-trimpath=$(GOPATH)'" "-gcflags=all='-trimpath=$(GOPATH)'"
$(warning $(GOFLAGS))

usage:

$ make -n build GOFLAGS=-tags=openssl
mk/golang.mk:15: -tags=openssl "-asmflags=all='-trimpath=/Users/oli/Code/go'" "-gcflags=all='-trimpath=/Users/oli/Code/go'"

$ make -n build
mk/golang.mk:15:  "-asmflags=all='-trimpath=/Users/oli/Code/go'" "-gcflags=all='-trimpath=/Users/oli/Code/go'"

No appending, just a default value

# Try to make building as reproducible as possible by stripping the go path.
GOFLAGS ?= "-asmflags=all='-trimpath=$(GOPATH)'" "-gcflags=all='-trimpath=$(GOPATH)'"
unexport GOFLAGS
$(warning $(GOFLAGS))

usage

$ make -n build GOFLAGS=-tags=openssl
mk/golang.mk:13: -tags=openssl

$ make -n build
mk/golang.mk:13: "-asmflags=all='-trimpath=/Users/oli/Code/go'" "-gcflags=all='-trimpath=/Users/oli/Code/go'"

[olizilla]

This change works as is, so is fine to merge, but I think we should fix the GOFLAGS definition as per #6843 (comment)

[Stebalien]

Yeah, I'll go through the variables in a followup PR.