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. The specific failures in these jobs happens due to the CMP0155
policy turning CMAKE_CXX_SCAN_FOR_MODULES on, which is turned off in the next
commit. But there are other things that could cause the threads package not to
be found, and there are platforms where it may not be required, so it is better
to make it an optional instead of required dependency.

Technically this change is not needed to make all CI jobs pass as long as the
next commit is present. But since this find_package error obscures other errors
that would be clearer, and since there are other conditions that could trigger
it, it is worth fixing generally. For example, this same failure also seems to
happen in the llvm job as well when CMP0137 is disabled or
CMAKE_TRY_COMPILE_NO_PLATFORM_VARIABLES is set to true.

Error looks like:

 + 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)

and inside the CMakeConfigureLog.yaml file there are "/bin/sh:
CMAKE_CXX_COMPILER_CLANG_SCAN_DEPS-NOTFOUND: not found" errors.

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)