Deactivate Kokkos mdspan impl if standard header is not found

CMakeLists.txt

@@ -6,6 +6,9 @@ list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
 # Forbids in-source builds
 include(CheckNotInSources)
 
+# Check for some headers
+include(CheckIncludeFileCXX)
+
 # use PkgConfig to find packages
 find_package(PkgConfig REQUIRED)
 
@@ -404,6 +407,13 @@ set(CMAKE_CXX_STANDARD "20")
 set(KOKKOS_SOURCE_DIR "${PUGS_SOURCE_DIR}/packages/kokkos")
 set(KOKKOS_BINARY_DIR "${PUGS_BINARY_DIR}/packages/kokkos")
 
+# test since mdspan is not supported by g++-10 and clang-11 (used in CI)
+check_include_file_cxx("mdspan" CXX_MDSPAN_SUPPORT)
+if (NOT DEFINED ${CXX_MDSPAN_SUPPORT})
+  message(STATUS "Kokkos_ENABLE_IMPL_MDSPAN is deactivated since standard library does not support mdspan")
+  set(Kokkos_ENABLE_IMPL_MDSPAN OFF CACHE BOOL "")
+endif()
+
 # disable Kokkos deprecation warnings by default
 set(Kokkos_ENABLE_DEPRECATION_WARNINGS OFF CACHE BOOL "")