For now meson build system determines automatically if zstd is available: https://gitlab.freedesktop.org/mesa/mesa/commit/a8d941091f72923561a6c58b46ccb264b6a0e205 So there should be a USE=zstd (or USE=+zstd, I don't know) for that.
The bug has been closed via the following commit(s): https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=83541cb2bb029fc1770632924386f2ba5954f352 commit 83541cb2bb029fc1770632924386f2ba5954f352 Author: Matt Turner <mattst88@gentoo.org> AuthorDate: 2020-02-19 18:20:25 +0000 Commit: Matt Turner <mattst88@gentoo.org> CommitDate: 2020-02-19 18:26:45 +0000 media-libs/mesa: Add IUSE=+zstd Closes: https://bugs.gentoo.org/709744 Signed-off-by: Matt Turner <mattst88@gentoo.org> media-libs/mesa/mesa-9999.ebuild | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)
20.0.0 will be released today, and it'll have IUSE=+zstd as well.