Makefile: switch to POSIX mode to handle errors

[thaJeztah]

similar to docker/docker-ce-packaging#510 and moby/sys#19 (comment)

relates to docker/docker-ce-packaging#504 (comment)

.POSIX

The application shall ensure that this special target is specified without prerequisites
or commands. If it appears as the first non-comment line in the makefile, make shall
process the makefile as specified by this section; otherwise, the behavior of make is
unspecified.

Running the makefile with .POSIX runs shells with the -e options, which helps with
handling errors; without this, make can complet "succesfully", even if shell commands
in a target fail.

Note that this patch does not change behavior on macOS, which runs an older version of
GNU make that does not support these options.

[thaJeztah]

@silvin-lubecki @tianon PTAL

[silvin-lubecki]

LGTM

[thaJeztah]

Rebased to check if this catches the failure we currently have in automatic runc-version detection because containerd "master" switched to go.mod, therefore these lines now fail;

containerd-packaging/common/common.mk

Lines 27 to 31 in 484fe7c

	RUNC_REF?=$(shell grep "opencontainers/runc" "$(CONTAINERD_DIR)/vendor.conf" | awk '{print $$2}')
	else
	# TODO adjust GOVERSION and RUNC_REF macro to take CONTAINERD_REMOTE into account
	GOVERSION?=$(shell curl -fsSL "https://raw.githubusercontent.com/containerd/containerd/$(REF)/contrib/Dockerfile.test" | grep "ARG GOLANG_VERSION" | awk -F'=' '{print $$2}')
	RUNC_REF?=$(shell curl -fsSL "https://raw.githubusercontent.com/containerd/containerd/$(REF)/vendor.conf" | grep "opencontainers/runc" | awk '{print $$2}')

[thaJeztah]

ok, that still didn't catch that problem; we can probably have a look at another approach for that