cmake: Fix openbsd/freebsd error caused by CMP0137

CMake 3.24 started passing more compiler flags to try-compiler targets as
described https://cmake.org/cmake/help/latest/policy/CMP0137.html. Revert to
previous behavior by setting CMAKE_TRY_COMPILE_NO_PLATFORM_VARIABLES

Error is:

 + cmake /home/runner/work/libmultiprocess/libmultiprocess -G Ninja
-- The CXX compiler identification is Clang 16.0.6
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD - Failed
-- Looking for pthread_create in pthreads
-- Looking for pthread_create in pthreads - not found
-- Looking for pthread_create in pthread
-- Looking for pthread_create in pthread - not found
-- Check if compiler accepts -pthread
-- Check if compiler accepts -pthread - no
CMake Error at /usr/local/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:233 (message):
  Could NOT find Threads (missing: Threads_FOUND)
Call Stack (most recent call first):
  /usr/local/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:603 (_FPHSA_FAILURE_MESSAGE)
  /usr/local/share/cmake/Modules/FindThreads.cmake:226 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
  CMakeLists.txt:41 (find_package)

Author: ryanofsky

cmake/compat_find.cmake

@@ -18,3 +18,9 @@
 # been noticed upstream because it only affects CMake packages depending on
 # capnproto, not autoconf packages.
 set(FOUND_LIBATOMIC TRUE)
+
+# Prevent env CXXFLAGS=-Werror from leaking into CMake's try-compile checks
+# and causing find_package(Threads REQUIRED) to fail on FreeBSD/OpenBSD.
+# In the future, prefer adding warning flags via an INTERFACE target rather
+# than environment variables.
+set(CMAKE_TRY_COMPILE_NO_PLATFORM_VARIABLES TRUE)