cmake: fix CMAKE_CXX_COMPILER_CLANG_SCAN_DEPS-NOTFOUND error with new cmake policies

With latest cmake policies, openbsd and freebsd CI jobs fail due to a lack of a
clang-scan-deps tool. The tool could potentially be installed on these
platforms but it is unclear how to do that and the project isn't using modules
anyway, so just disable them here. Errors look like:

+ cmake --build . --parallel -t all tests mpexamples -- -k 0
[1/114] Scanning /home/runner/work/libmultiprocess/libmultiprocess/src/mp/util.cpp for CXX dependencies
FAILED: CMakeFiles/mputil.dir/src/mp/util.cpp.o.ddi
"CMAKE_CXX_COMPILER_CLANG_SCAN_DEPS-NOTFOUND" -format=p1689 -- /usr/bin/c++  -I/home/runner/work/libmultiprocess/libmultiprocess/include -I/home/runner/work/libmultiprocess/libmultiprocess/build-openbsd/include -isystem /usr/local/include -Werror -Wall -Wextra -Wpedantic -Wno-unused-parameter -std=gnu++20 -x c++ /home/runner/work/libmultiprocess/libmultiprocess/src/mp/util.cpp -c -o CMakeFiles/mputil.dir/src/mp/util.cpp.o -resource-dir "/usr/lib/clang/16" -MT CMakeFiles/mputil.dir/src/mp/util.cpp.o.ddi -MD -MF CMakeFiles/mputil.dir/src/mp/util.cpp.o.ddi.d > CMakeFiles/mputil.dir/src/mp/util.cpp.o.ddi.tmp && mv CMakeFiles/mputil.dir/src/mp/util.cpp.o.ddi.tmp CMakeFiles/mputil.dir/src/mp/util.cpp.o.ddi
/bin/sh: CMAKE_CXX_COMPILER_CLANG_SCAN_DEPS-NOTFOUND: not found

Author: ryanofsky

CMakeLists.txt

@@ -10,6 +10,11 @@ if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   set(CMAKE_CXX_STANDARD_REQUIRED YES)
 endif()
 
+# Disable automatic C++20 module dependency scanning.
+# CMake >=3.28 tries to use `clang-scan-deps` by default, which may not
+# be installed on all platforms. We don't use named modules, so turn this off.
+set(CMAKE_CXX_SCAN_FOR_MODULES OFF)
+
 include("cmake/compat_find.cmake")
 
 add_library(mpdeps INTERFACE)