Update variables override mechanism to append

[stanislav-zaprudskiy]

The updated mechanism brought by #362 behaves differently to the behavior in #352 in a way that it prepends to the variable not appends to it when overriding with environment - which makes the overall feature of "adding to a variable" less useful.

The PRs reverts #362 and configures Makefile to not produce warnings on unused variables by default:

• Remove MAKEFLAGS=--warn-undefined-variables
  This is not generally required, and could manually be added as command
  line argumen when needed e.g. to debug a Makefile.

  `--warn-undefined-variables'

  Issue a warning message whenever make sees a reference to an undefined
  variable. This can be helpful when you are trying to debug makefiles
  which use variables in complex ways.

---

A practical example. The following configuration works perfectly fine both in regular CI workflow and on local machine:

variables:
  GO_TESTFLAGS: "-short -count=1 -v -failfast"

but making it execute with -short=false (e.g. in a nightly CI workflow) requires providing all other arguments as well, as += prepends to a variable, not appends to it as seen below.

---

Given the following Makefile:

GO_TESTFLAGS += -short=true

foo:
	@echo flags are \"$(GO_TESTFLAGS)\"

it produces:

❯ GO_TESTFLAGS="-short=false" make
flags are "-short=false -short=true"

Whereas a Makefile updated to

GO_TESTFLAGS := -short=true $(GO_TESTFLAGS)
...

produces:

❯ GO_TESTFLAGS="-short=false" make
flags are "-short=true -short=false"

which would result to an overridden argument when used with e.g. go test.

[github-actions]

Merging this branch will increase overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/sapcc/go-makefile-maker/internal/makefile	8.02% (+0.02%)	👍

---

Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/sapcc/go-makefile-maker/internal/makefile/makefile.go	0.00% (ø)	195 (-1)	0	195 (-1)

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.

[SuperSandro2000]

LGTM

[majewsky]

I object to this being removed. Let's please figure out a way to bring this back, since this has already caught bugs in the past.

[majewsky]

Reverting this entire PR via #373 because it breaks the documented behavior. make GO_TESTFLAGS="-short=false" instead of GO_TESTFLAGS="-short=false" make would have produced the desired effect, and that was already documented.