This was added in 2012-01 in response to http://debbugs.gnu.org/10552 .
However, this issue had already been fixed in the trunk two years
earlier, by removing POSIX_SIGNALS altogether. Ref:
http://lists.gnu.org/archive/html/emacs-devel/2010-04/msg00002.html
+2012-06-13 Glenn Morris <rgm@gnu.org>
+
+ * s/gnu.h (POSIX_SIGNALS): Remove (not used since 2010-05-04).
+
2012-06-13 Paul Eggert <eggert@cs.ucla.edu>
USE_LISP_UNION_TYPE + USE_LSB_TAG cleanup (Bug#11604)
((FILE)->_IO_write_ptr - (FILE)->_IO_write_base)
#endif /* !_IO_STDIO_H */
#endif /* emacs */
-
-#define POSIX_SIGNALS 1