Add a GNU-compatible TEMP_FAILURE_RETRY.

I wondered about #ifndef, but the other macros in here don't use it.

I also wondered about __GNUC__, since this macro uses two GCC extensions.

libc/include/unistd.h

@@ -185,6 +185,14 @@ extern int cacheflush(long start, long end, long flags);
 extern pid_t tcgetpgrp(int fd);
 extern int   tcsetpgrp(int fd, pid_t _pid);
 
+/* Used to retry syscalls that can return EINTR. */
+#define TEMP_FAILURE_RETRY(exp) ({         \
+    typeof (exp) _rc;                      \
+    do {                                   \
+        _rc = (exp);                       \
+    } while (_rc == -1 && errno == EINTR); \
+    _rc; })
+
 __END_DECLS
 
 #endif /* _UNISTD_H_ */