Unbreak MinGW64 build with pthreads.h installed

* nt/mingw-cfg.site (gl_cv_func_pthread_sigmask_macro): Set to
"no", to avoid compiling Gnulib's pthread_sigmask.c.
Reported by Richard Copley <rcopley@gmail.com>.
1 job for master in 65 minutes and 37 seconds (queued for 5 seconds)
Status Job ID Name Coverage
failed #1954


Name Stage Failure
test-all Test
make[2]: *** [check-doit] Error 1
Makefile:318: recipe for target 'check-doit' failed
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:293: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:957: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1