Update SWIG flags for OpenBSD

[ron-at-swgy]

Fixes #4249

This commit adds an OpenBSD specific flag for use with SWIG,
-DSMALL_LONG, and further specifies the -DSWIGWORDSIZE flag
conditions to exclude OpenBSD (as is done with Apple).

Validation of CMake platform check:

message(STATUS "system name: ${CMAKE_SYSTEM_NAME}")

if(CMAKE_SYSTEM_NAME STREQUAL "OpenBSD")
  message(STATUS "On OpenBSD")
  list(APPEND CMAKE_SWIG_FLAGS "-DSMALL_LONG")
else()
  message(STATUS "NOT on OpenBSD")
endif()

message(FATAL_ERROR "stop cmake configure to fast iterate ! (to remove after dev/debug)")

-- system name: OpenBSD
-- On OpenBSD
CMake Error at cmake/python.cmake:43 (message):
  stop cmake configure to fast iterate ! (to remove after dev/debug)
  Call Stack (most recent call first):
    CMakeLists.txt:429 (include)

Result

This change, combined with #4259 and #4257 allows me to build the .whl file for
the python or-tools package. I then validated the CBC_MIXED_INTEGER_PROGRAMMING
and GLOP solvers with a simple linear programming problem in the Jupyter Lab
environment.

Installing collected packages: immutabledict, absl-py, ortools
Successfully installed absl-py-2.1.0 immutabledict-4.2.0 ortools-9.10.9999

[ron-at-swgy]

@Mizux the build completed successful without SMALL_LONG defined.