Install pollstl headers

Some macros don't work when the source directory with the headers isn't available.

added BugFix Build System Externals labels

requested review from @p.-a.loizeau

assigned to @f.uhlig

mentioned in merge request !1794 (merged)

This MR fixes an issue discussed in !1794 (merged).

approved this merge request

Approving as verified by user in !1794 (comment 46305) + diff making sense with the external repo structure

enabled an automatic merge when the pipeline for f71b200e succeeds

Dear @f.uhlig,

you have been identified as code owner of at least one file which was changed with this merge request.

Please check the changes and approve them or request changes.

added CodeOwners label

merged