Merge branch 'fix-prepare-build-dir' into 'master'

[tools] Fix "unbound variable" error in prepare-build-dir

See merge request flowpack/manycore/emper!19

tools/prepare-build-dir

@@ -116,6 +116,6 @@ if grep -F "WARNING: Unknown options:" "${LOGFILE}"; then
 	exit 1
 fi
 
-if [[ ${GITLAB_CI} ]]; then
+if [[ -v GITLAB_CI ]]; then
 	meson configure --no-pager "${ABSOLUTE_BUILDDIR}"
 fi