Emacs master fails to build in MinGW-W64 (MSYS2).

The cause seems to be that since the configuration changes in commit "Update from Gnulib" of Wed May 29 16:33:51 2019 -0700, we now include pthread_signal.h (<https://sourceforge.net/p/mingw-w64/mingw-w64/ci/master/tree/mingw-w64-libraries/winpthreads/include/pthread_signal.h>) which does this:

#define pthread_sigmask(H, S1, S2) 0

The result:

gcc -c -mtune=generic -DUSE_CRT_DLL=1 -I /c/projects/emacs/nt/inc -mtune=generic -MMD -MF deps/pthread_sigmask.d -MP -fno-common -O2 -I. -I../src -I. -I./../src pthread_sigmask.c
In file included from C:/msys64/mingw64/x86_64-w64-mingw32/include/signal.h:10,
from pthread_sigmask.c:20:
pthread_sigmask.c:34:1: error: expected identifier or '(' before numeric constant
34 | pthread_sigmask (int how, const sigset_t *new_mask, sigset_t *old_mask)

| ^~~~~~~~~~~~~~~