If this isn't set and you have postegresql installed in your chroot it will actually build and link with it, causing a so called "automagic dependency" https://github.com/gentoo/gentoo/pull/6171
The bug has been referenced in the following commit(s): https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=9a14533e0a70318885b30dd89e877af4b87a3335 commit 9a14533e0a70318885b30dd89e877af4b87a3335 Author: Aaron W. Swenson <titanofold@gentoo.org> AuthorDate: 2017-11-12 17:23:57 +0000 Commit: Aaron W. Swenson <titanofold@gentoo.org> CommitDate: 2017-11-12 17:26:28 +0000 games-action/minetest: Add PgSQL dependency Add USE flag to control dependency on PostgreSQL. Bug: https://bugs.gentoo.org/637134 Package-Manager: Portage-2.3.8, Repoman-2.3.3 games-action/minetest/metadata.xml | 1 + games-action/minetest/minetest-0.4.16-r1.ebuild | 134 ++++++++++++++++++++++++ 2 files changed, 135 insertions(+)}