pthread_sigmask() on windows (wine actualy)

I got this while compiling master today using mingw32 cross toolchain

    CC run-command.o
run-command.c: In function 'run_thread':
run-command.c:596:3: warning: implicit declaration of function
'pthread_sigmask' [-Wimplicit-function-declaration]
run-command.c:596:23: error: 'SIG_BLOCK' undeclared (first use in this function)
run-command.c:596:23: note: each undeclared identifier is reported
only once for each function it appears in

This is added in c792d7b (run-command: teach async threads to ignore SIGPIPE - 2016-04-19). From the look of it, wrapping #ifndef GIT_WINDOWS_NATIVE is probably enough? I checked gfw/master too to see how gfw deals with it, but the commit has not been merged there yet.