MDEV-27686 Moving libexecinfo out of FreeBSD / OpenBSD specific CMake code

On all Unix platforms, link libexecinfo as system library,
if it contains backtrace_symbols_fd function, and libc does not contain
this function

Also remove cmake/os/OpenBSD.cmake, as after the fix it serves no purpose.

cmake/os/FreeBSD.cmake

@@ -28,8 +28,3 @@ SET(EXECINFO_ROOT /usr/local CACHE INTERNAL "Where to find execinfo library and
 INCLUDE_DIRECTORIES(${EXECINFO_ROOT}/include)
 SET(CMAKE_REQUIRED_INCLUDES ${CMAKE_REQUIRED_INCLUDES} ${EXECINFO_ROOT}/include)
 SET(ENV{LIB} "$ENV{LIB}:${EXECINFO_ROOT}/lib")
-FIND_LIBRARY(EXECINFO NAMES execinfo)
-IF(EXECINFO)
- SET(LIBEXECINFO ${EXECINFO})
-ENDIF()
-

cmake/os/OpenBSD.cmake

-# Copyright (C) 2012 Monty Program Ab, 2021 Brad Smith
-# 
-# This program is free software; you can redistribute it and/or modify
-# it under the terms of the GNU General Public License as published by
-# the Free Software Foundation; version 2 of the License.
-#
-# This program is distributed in the hope that it will be useful,
-# but WITHOUT ANY WARRANTY; without even the implied warranty of
-# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-# GNU General Public License for more details.
-#
-# You should have received a copy of the GNU General Public License
-# along with this program; if not, write to the Free Software
-# Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1335  USA 
-
-# This file includes OpenBSD specific options and quirks, related to system checks
-
-# Find libexecinfo (library that contains backtrace_symbols etc)
-FIND_LIBRARY(EXECINFO NAMES execinfo)
-IF(EXECINFO)
- SET(LIBEXECINFO ${EXECINFO})
-ENDIF()

configure.cmake

@@ -134,6 +134,8 @@ IF(UNIX)
   IF(NOT LIBRT)
     MY_SEARCH_LIBS(clock_gettime rt LIBRT)
   ENDIF()
+  MY_SEARCH_LIBS(backtrace_symbols_fd execinfo LIBEXECINFO)
+
   set(THREADS_PREFER_PTHREAD_FLAG ON)
   FIND_PACKAGE(Threads)