app-portage/prefix-toolkit(winnt): pull pthreads4w, intercept -pthread

Some X11 library build system is dumb enough to just use -pthread,
while others do triage this, before trying with -lpthread.  So we
intercept the -pthread compiler flag and add -lpthread instead.
We cannot do that in parity because pthreads4w is installed in the
stacked prefix, where it is built using parity.

Package-Manager: Portage-2.3.84, Repoman-2.3.20
Signed-off-by: Michael Haubenwallner <[email protected]>