Revert "build: removed unused enum that evaluates to empty on NetBSD."

This reverts commit 5ca5bed0.

The enum as such is not used, but its values are...

Fixes compilation errors on Solaris and other platforms without accept4(2).

src/fd_control.h

@@ -11,6 +11,31 @@
 #ifdef HAVE_SYS_SOCKET_H
 #include <sys/socket.h>
 #endif
+
+/* NB: The default values are compatible with Linux,
+ *     but should work on other OSs as well, since
+ *     accept4(2) is emulated by us anyway.
+ */
+enum pike_sock_flags {
+#ifndef SOCK_CLOEXEC
+#if !defined(SOCK_NONBLOCK) || (SOCK_NONBLOCK != 0x80000)
+  SOCK_CLOEXEC = 0x80000,
+#else
+  /* Unlikely, but... */
+  SOCK_CLOEXEC = 0x40000,
+#endif /* !SOCK_NONBLOCK || SOCK_NONBLOCK != 0x80000 */
+#define SOCK_CLOEXEC	SOCK_CLOEXEC
+#endif
+#ifndef SOCK_NONBLOCK
+#if (SOCK_CLOEXEC != 0x00800)
+  SOCK_NONBLOCK = 0x00800,
+#else
+  /* Unlikely, but... */
+  SOCK_NONBLOCK = 0x00400,
+#endif
+#define SOCK_NONBLOCK	SOCK_NONBLOCK
+#endif
+};
 #endif /* !HAVE_ACCEPT4 */
 
 /* Prototypes begin here */