add pthread as a dependency of a static lib

Downloaded from https://trac.netlabs.org/ports/changeset/2220
10 jobs for pthread-static in 10 minutes and 5 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Build
passed #4600641
gst-macos-10.15
meson macos

00:01:10

passed #4600638
meson-mingw-w64-build

00:07:07

manual #4600642
docker windows 1809 allowed to fail manual
meson msys2
passed #4600636
meson-shared-build

00:07:15

passed #4600637
meson-static-build

00:07:28

passed #4600639
docker windows 1809
meson vs2017 amd64

00:01:12

passed #4600640
docker windows 1809
meson vs2017 x86

00:01:12

passed #4600635
mingw-build

00:08:32

passed #4600633
shared-build

00:10:05

passed #4600634
static-build

00:08:36