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name: HAVE_SO_BINDTODEVICE referenced before it has a chance to be defined

Describe the bug

In includes/osdep.h, HAVE_SO_BINDTODEVICE is referenced on line 152 in the following conditional:

#if defined (USE_BPF_SEND) || defined (USE_NIT_SEND) || \
    defined (USE_DLPI_SEND) || defined (USE_UPF_SEND) || \
    defined (USE_LPF_SEND) || \
    (defined (USE_SOCKET_SEND) && defined (HAVE_SO_BINDTODEVICE))
#  define USE_SOCKET_FALLBACK
#  define USE_FALLBACK
#endif

However, it might not get defined until line 267, as follows:

#if defined (SO_BINDTODEVICE) && !defined (HAVE_SO_BINDTODEVICE)
# define HAVE_SO_BINDTODEVICE
#endif

Therefore, if USE_SOCKET_SEND is defined, it is possible that USE_SOCKET_FALLBACK and USE_FALLBACK will not get defined, even though they should, simply because HAVE_SO_BINDTODEVICE is referenced before it has had a chance to get defined.

To Reproduce

Steps to reproduce the behavior:

1. Add a #pragma message() line inside the first #if block mentioned above (e.g., at line 153 of includes/osdep.h) so that the preprocessor will print the message if it enters the block
2. Compile isc-dhcp on a system which defines SO_BINDTODEVICE (e.g., Debian Stretch), while also defining USE_SOCKETS (e.g., via the --enable-use-sockets configure flag)
3. Note that the message does not print, although it should

Expected behavior

HAVE_SO_BINDTODEVICE should have an opportunity to be defined before it is referenced

Environment:

• ISC DHCP version: All versions since commit d758ad8c (pre-version 4.0.0)
• OS: Debian Jessie/Stretch, most likely many other Linux flavors
• Which features were compiled in: USE_SOCKETS

Describe the solution you'd like

Reorder the file includes/osdep.h to ensure HAVE_SO_BINDTODEVICE has a chance to be defined before it is referenced.