[regression between 4.6.0 and 4.9.0] Build breaks on FreeBSD/arm64: modules/core/src/system.cpp:573:13: error: unknown type

[yurivict]

System Information

OpenCV version: 4.9.0
FreeBSD 15

Detailed description

/construction/xports/graphics/opencv/work/opencv-4.9.0/modules/core/src/system.cpp:573:13: error: unknown type 
name 'Elf64_auxv_t'
           Elf64_auxv_t auxv;

The #include <gelf.h> statement for Elf64_auxv_t is missing.
The search for the package elfutils which installs the header gelf.h is also missing.

The code section beginning with int cpufile = open("/proc/self/auxv", O_RDONLY); is new in 4.9.0

Additionally, when the file /proc/self/auxv doesn't exist on FreeBSD, it should print that linprocfs should be enabled, because /proc/self/auxv is only created when linprocfs is enabled. The current code is silent in case of the failure to open this file.

In short, 3 items need to be fixed:

1. Add the include for Elf64_auxv_t
2. Add search for the elfutils in configure
3. Add the user warning when "/proc/self/auxv" isn't available on FreeBSD suggesting to enable linprocfs

Steps to reproduce

n/a

Issue submission checklist

• I report the issue, it's not a question
• I checked the problem with documentation, FAQ, open issues, forum.opencv.org, Stack Overflow, etc and have not found any solution
• I updated to the latest OpenCV version and the issue is still there
• There is reproducer code and related data files (videos, images, onnx, etc)

[asmorkalov]

Unfortunately BSD systems are out for OpenCV team CI Scope. Feel free to contribute solution.