[PATCH v2] Windows: only add a no-op pthread_sigmask() when needed

In f924b52 (Windows: add pthread_sigmask() that does nothing, 2016-05-01), we introduced a no-op for Windows. However, this breaks building Git in Git for Windows' SDK because pthread_sigmask() is already a no-op there, #define'd in the pthread_signal.h header in /mingw64/x86_64-w64-mingw32/include/.

Let's wrap the definition of pthread_sigmask() in a guard that skips it when compiling with MinGW-w64' headers.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
Published-As: https://github.com/dscho/git/releases/tag/mingw-sigmask-v2
 compat/win32/pthread.h | 2 ++
 1 file changed, 2 insertions(+)
Interdiff vs v1:

 diff --git a/compat/win32/pthread.h b/compat/win32/pthread.h
 index 8df702c..1c16408 100644
 --- a/compat/win32/pthread.h
 +++ b/compat/win32/pthread.h
 @@ -104,7 +104,7 @@ static inline void *pthread_getspecific(pthread_key_t key)
  	return TlsGetValue(key);
  }
  
 -#ifndef pthread_sigmask
 +#ifndef __MINGW64_VERSION_MAJOR
  static inline int pthread_sigmask(int how, const sigset_t *set, sigset_t *oset)
  {
  	return 0;