cmake: fix find_package(Threads REQUIRED) error after updating cmake policy

find_package(Threads REQUIRED) started failing in freebsd, openbsd and llvm
builds after previous change increasing cmake policy version. Exact reason is
not clear and setting cmake_policy(SET CMP0155 OLD) made it work again, but it
makes sense for a separate threading library to not be required so this change
makes it optional and seems to fix the builds. 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

@@ -38,7 +38,14 @@ 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
@@ -228,7 +235,7 @@ 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)

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)