contrib: pthread: don't rebuild winpthread if it's in the toolchain
We don't have any local patch for this library.
A normal mingw-w64 install has both libpthread.a and libpthread.dll.a, contribs will use the former.
We will still need to download the package to patch the dcomp.h
file. Hopefully the contrib dependencies won't consider it a (modified) dependency.
Edited by Steve Lhomme