app-arch/zstd: fix underlinking against pthreads, bug #713940

app-arch/zstd: fix underlinking against pthreads, bug #713940

buscher noticed late gcc-10 link failure which exposed an
underlinked libzstd.so when built with multithreaded support.

The change adds an ebuild workaround to compile/link against
-pthread on platforms where this options exist. Real fix is
left to upstream.

Reported-by: buscher on #gentoo-toolchain
Closes: https://bugs.gentoo.org/713940
Package-Manager: Portage-2.3.94, Repoman-2.3.21
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>