Fixes build issue with LLVM on Windows with cmake (#10776)

LLVM fails to build on Windows with C++ 20 standard.  When building LLVM
on Windows, we use C++ 14 already.  This commit changes the c++ standard
to be picked by each subdirectory rather than the standard being picked
by the root level cmake file.

3rdparty/CMakeLists.txt

@@ -2,6 +2,8 @@ find_package(PkgConfig)
 include(ExternalProject)
 include(CMakeDependentOption)
 
+set(CMAKE_CXX_STANDARD 20)
+
 # Dummy target to use when lib isn't available
 add_library(3rdparty_dummy_lib INTERFACE)
 

CMakeLists.txt

@@ -26,7 +26,6 @@ option(USE_PRECOMPILED_HEADERS "Use precompiled headers" OFF)
 
 set(CMAKE_MODULE_PATH "${CMAKE_CURRENT_LIST_DIR}/buildfiles/cmake")
 
-set(CMAKE_CXX_STANDARD 20)
 include(CheckCXXCompilerFlag)
 
 if(NOT CMAKE_BUILD_TYPE)

rpcs3/CMakeLists.txt

@@ -10,6 +10,8 @@ include(${CMAKE_CURRENT_SOURCE_DIR}/git-version.cmake)
 include(ConfigureCompiler)
 include(CheckFunctionExists)
 
+set(CMAKE_CXX_STANDARD 20)
+
 set(ADDITIONAL_LIBS "")
 if(CMAKE_SYSTEM MATCHES "Linux")
     #on some Linux distros shm_unlink and similar functions are in librt only