Fixes sys/select.h when the SDK does not define fd_set.

include/esp8266/sys/select.h

@@ -34,12 +34,53 @@
 #ifndef _SYS_SELECT_H_
 #define _SYS_SELECT_H_
 
-/* We can actually pull the FD Set macros from here */
+/* We can actually pull the FD Set macros from here, but sometimes that won't
+ * work. */
 #include <sys/types.h>
 #include <sys/time.h>
 
 #ifdef __cplusplus
 extern "C" {
+
+
+#ifndef _SYS_TYPES_FD_SET
+#  define _SYS_TYPES_FD_SET
+#  define	NBBY	8		/* number of bits in a byte */
+/*
+ * Select uses bit masks of file descriptors in longs.
+ * These macros manipulate such bit fields (the filesystem macros use chars).
+ * FD_SETSIZE may be defined by the user, but the default here
+ * should be >= NOFILE (param.h).
+ */
+#  ifndef	FD_SETSIZE
+#	define	FD_SETSIZE	64
+#  endif
+
+typedef	long	fd_mask;
+#  define	NFDBITS	(sizeof (fd_mask) * NBBY)	/* bits per mask */
+#  ifndef	howmany
+#	define	howmany(x,y)	(((x)+((y)-1))/(y))
+#  endif
+
+/* We use a macro for fd_set so that including Sockets.h afterwards
+   can work.  */
+typedef	struct _types_fd_set {
+	fd_mask	fds_bits[howmany(FD_SETSIZE, NFDBITS)];
+} _types_fd_set;
+
+#define fd_set _types_fd_set
+
+#  define	FD_SET(n, p)	((p)->fds_bits[(n)/NFDBITS] |= (1L << ((n) % NFDBITS)))
+#  define	FD_CLR(n, p)	((p)->fds_bits[(n)/NFDBITS] &= ~(1L << ((n) % NFDBITS)))
+#  define	FD_ISSET(n, p)	((p)->fds_bits[(n)/NFDBITS] & (1L << ((n) % NFDBITS)))
+#  define	FD_ZERO(p)	(__extension__ (void)({ \
+     size_t __i; \
+     char *__tmp = (char *)p; \
+     for (__i = 0; __i < sizeof (*(p)); ++__i) \
+       *__tmp++ = 0; \
+}))
+#endif // fd_set
+
 #endif
 
 /** POSIX select().