cmake: fix find_package(Threads REQUIRED) error with new cmake policies

With latest cmake policies, find_package(Threads REQUIRED) fails in freebsd and
openbsd builds. Exact reason is not clear, but it makes sense for a separate
threading library to not be required, so this change makes the package optional
and seems to fix the builds. Can maybe debug actual reason this started failing
later.

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

CMakeLists.txt

@@ -12,7 +12,14 @@ endif()
 
 include("cmake/compat_find.cmake")
 
-find_package(Threads REQUIRED)
+add_library(mpdeps INTERFACE)
+
+find_package(Threads)
+
+if(Threads_FOUND)
+  target_link_libraries(mpdeps INTERFACE Threads::Threads)
+endif()
+
 find_package(CapnProto 0.7 QUIET NO_MODULE)
  if(NOT CapnProto_FOUND)
    message(FATAL_ERROR
@@ -202,7 +209,7 @@ target_link_libraries(mpgen PRIVATE CapnProto::capnp)
 target_link_libraries(mpgen PRIVATE CapnProto::capnp-rpc)
 target_link_libraries(mpgen PRIVATE CapnProto::capnpc)
 target_link_libraries(mpgen PRIVATE CapnProto::kj)
-target_link_libraries(mpgen PRIVATE Threads::Threads)
+target_link_libraries(mpgen PRIVATE mpdeps)
 set_target_properties(mpgen PROPERTIES
     INSTALL_RPATH_USE_LINK_PATH TRUE)
 set_target_properties(mpgen PROPERTIES

cmake/pthread_checks.cmake

@@ -9,7 +9,7 @@ include(CMakePushCheckState)
 include(CheckCXXSourceCompiles)
 
 cmake_push_check_state()
-set(CMAKE_REQUIRED_LIBRARIES Threads::Threads)
+set(CMAKE_REQUIRED_LIBRARIES mpdeps)
 check_cxx_source_compiles("
   #include <pthread.h>
   int main(int argc, char** argv)

example/CMakeLists.txt

@@ -9,20 +9,20 @@ add_executable(mpcalculator
 )
 target_capnp_sources(mpcalculator ${CMAKE_CURRENT_SOURCE_DIR} init.capnp calculator.capnp printer.capnp)
 target_include_directories(mpcalculator PRIVATE ${CMAKE_CURRENT_SOURCE_DIR})
-target_link_libraries(mpcalculator PRIVATE Threads::Threads)
+target_link_libraries(mpcalculator PRIVATE mpdeps)
 
 add_executable(mpprinter
   printer.cpp
 )
 target_capnp_sources(mpprinter ${CMAKE_CURRENT_SOURCE_DIR} init.capnp calculator.capnp printer.capnp)
 target_include_directories(mpprinter PRIVATE ${CMAKE_CURRENT_SOURCE_DIR})
-target_link_libraries(mpprinter PRIVATE Threads::Threads)
+target_link_libraries(mpprinter PRIVATE mpdeps)
 
 add_executable(mpexample
   example.cpp
 )
 target_capnp_sources(mpexample ${CMAKE_CURRENT_SOURCE_DIR} init.capnp calculator.capnp printer.capnp)
 target_include_directories(mpexample PRIVATE ${CMAKE_CURRENT_SOURCE_DIR})
-target_link_libraries(mpexample PRIVATE Threads::Threads)
+target_link_libraries(mpexample PRIVATE mpdeps)
 
 add_custom_target(mpexamples DEPENDS mpexample mpcalculator mpprinter)

src/mp/util.cpp

@@ -2,14 +2,14 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
-#include <mp/config.h>
+#include <mp/config.h> // IWYU pragma: keep
 #include <mp/util.h>
 
 #include <cerrno>
 #include <cstdio>
 #include <kj/common.h>
 #include <kj/string-tree.h>
-#include <pthread.h>
+#include <pthread.h> // IWYU pragma: keep
 #include <sstream>
 #include <string>
 #include <sys/resource.h>

test/CMakeLists.txt

@@ -32,7 +32,7 @@ if(BUILD_TESTING AND TARGET CapnProto::kj-test)
   target_capnp_sources(mptest ${CMAKE_CURRENT_SOURCE_DIR} mp/test/foo.capnp)
   target_include_directories(mptest PRIVATE ${CMAKE_CURRENT_SOURCE_DIR})
   target_link_libraries(mptest PRIVATE CapnProto::kj-test)
-  target_link_libraries(mptest PRIVATE Threads::Threads)
+  target_link_libraries(mptest PRIVATE mpdeps)
 
   add_dependencies(mptests mptest)
   add_test(NAME mptest COMMAND mptest)