Updates for OpenBSD build support

[ron-at-swgy]

This pull request adds several references to the system identifier __OpenBSD__ in
checks where __FreeBSD__ is included. I have validated that each of the FreeBSD
checks are also appropriate for OpenBSD.

It should be noted that I'm still working through a completely successful build, but these changes are necessary to make incremental progress towards that goal.

First, the fenv structure definition referenced in ortools/util/fp_utils.h

fenv in fp_utils

The fenv struct is similar in shape to that of FreeBSD. For reference, the official CVSWeb repository shows a definition of:

/*
 * fenv_t represents the entire floating-point environment.
 */
typedef	struct {
	struct {
		unsigned int __control;		/* Control word register */
		unsigned int __status;		/* Status word register */
		unsigned int __tag;		/* Tag word register */
		unsigned int __others[4];	/* EIP, Pointer Selector, etc */
	} __x87;
	unsigned int __mxcsr;			/* Control, status register */
} fenv_t;

This is in alignment with that of the FreeBSD project.

Changes in sysinfo.cc

The file ortools/base/sysinfo.cc uses a number of system calls and
expects a /proc filesystem in some cases. Like FreeBSD, OpenBSD also
defines getrusage in sys/resource.h, has no proc filesystem, and
needs to include sys/time.h.

References:

• getrusage(2)

Update to python package

The ortools/python/setup.py.in file has been updated to include OpenBSD
as a classified for the package targets.

Update zvector

The ortools/util/zvector.h header should include machine/endian.h,
just like FreeBSD.

References:

• CVSWeb endian.h

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