Add a CMake property for c++ standard (17 or 20) (#2736)

Configure C++ standard with a CMake variable.

Defaults to C++20, but can be set to C++17  to test backwards compatibility.

* Add validation for allowed C++ standards.

* build CK in rehl8 docker with std=c++17

---------

Co-authored-by: illsilin_amdeng <Illia.Silin@amd.com>

[ROCm/composable_kernel commit: 99d27aca17]

CMakeLists.txt

@@ -16,6 +16,15 @@ else()
             "Choose the type of build, options are: None Debug Release RelWithDebInfo MinSizeRel.")
 endif()
 
+# Allow user to specify the C++ standard.
+# We must support C++17 builds until downstream users are migrated to C++20, but we default to C++20.
+set(CK_CXX_STANDARD "20" CACHE STRING "C++ standard to use (e.g. 17 or 20)")
+set(valid_cxx_standards 17 20)
+set_property(CACHE CK_CXX_STANDARD PROPERTY STRINGS ${valid_cxx_standards})
+if(NOT CK_CXX_STANDARD IN_LIST valid_cxx_standards)
+    message(FATAL_ERROR "CK_CXX_STANDARD must be one of ${valid_cxx_standards}")
+endif()
+
 # Default installation path
 if(NOT WIN32)
     set(CMAKE_INSTALL_PREFIX "/opt/rocm" CACHE PATH "")
@@ -345,7 +354,7 @@ find_package(Threads REQUIRED)
 link_libraries(Threads::Threads)
 
 ## C++
-set(CMAKE_CXX_STANDARD 20)
+set(CMAKE_CXX_STANDARD ${CK_CXX_STANDARD})
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_CXX_EXTENSIONS OFF)
 message(STATUS "CMAKE_CXX_COMPILER: ${CMAKE_CXX_COMPILER}")

Jenkinsfile

@@ -1358,6 +1358,7 @@ pipeline {
                         def docker_name = "${env.CK_DOCKERHUB_PRIVATE}:ck_rhel8_rocm6.3"
                         setup_args = """ -DGPU_TARGETS="gfx942" \
                                          -DCMAKE_CXX_FLAGS=" -O3 " \
+                                         -DCK_CXX_STANDARD="17" \
                                          -DCK_USE_ALTERNATIVE_PYTHON=/opt/Python-3.8.13/bin/python3.8 """
                         execute_args = " "
                     }