use systemd-detect-virt to detect building as root in container

Drop the older container detection attempts because they are not
reliable to detect being run as root in a container in github actions.

<cloph> https://github.com/google/oss-fuzz/issues/4093#issuecomment-656290094 "It appears some GitHub actions are run with docker.service (where docker is in /proc/self/cgroup) while the zstd actions are run with containerd.service where /proc/self/cgroup looks like […]"
<cloph> https://github.com/google/oss-fuzz/pull/4101 → probably also just use systemd-detect-virt instead of the grepping ourselves...

if we're root and systemd-detect-virt doesn't exist or it claims
we're not in a container then continue to abort the build

using LIB_FUZZING_ENGINE for the oss-fuzz specific case worked fine,
but lets try something a little more generic.

Change-Id: I59711b01dfcd052b5af899ad41ae5890f849eacb
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/113738
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>

Makefile.in

@@ -59,11 +59,14 @@ else # MAKE_RESTARTS
 .DEFAULT_GOAL := build
 
 check-if-root:
-	@if test ! `uname` = 'Haiku' -a `id -u` = 0 && test -z $$LIB_FUZZING_ENGINE && test -z $$container && ! grep -q 'lxc\|docker' /proc/self/cgroup && ! grep -q 'libpod_parent' /proc/self/cgroup; then \
+	@if test ! `uname` = 'Haiku' -a `id -u` = 0; then \
+		# if the systemd-detect-virt command doesn't exist or it reports we're not in a container \
+		if test ! -x "$$(command -v systemd-detect-virt)" || ! systemd-detect-virt -c -q; then \
 			echo; \
 			echo 'Building LibreOffice as root is a very bad idea, use a regular user.'; \
 			echo; \
 			exit 1; \
+		fi \
 	fi
 
 gb_Side ?= host