Merge pull request #1280 from dutkalex/patch-2

Update CMakeLists.txt

CMakeLists.txt

@@ -46,11 +46,19 @@ if(NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
     "Debug" "Release" "RelWithDebInfo")
 endif()
 
-set( CMAKE_C_STANDARD 11 )
+if( NOT DEFINED CMAKE_C_STANDARD ) 
+  set( CMAKE_C_STANDARD 11 )
+elseif( CMAKE_C_STANDARD LESS 11 )
+  message( FATAL_ERROR "The CMAKE_C_STANDARD variable has been set to ${CMAKE_C_STANDARD}, but t8code requires C11" )
+endif()
 set( CMAKE_C_STANDARD_REQUIRED ON )
 set( CMAKE_C_EXTENSIONS OFF )
 
-set( CMAKE_CXX_STANDARD 17 )
+if( NOT DEFINED CMAKE_CXX_STANDARD ) 
+  set( CMAKE_CXX_STANDARD 17 )
+elseif( CMAKE_CXX_STANDARD LESS 17 )
+  message( FATAL_ERROR "The CMAKE_CXX_STANDARD variable has been set to ${CMAKE_CXX_STANDARD}, but t8code requires C++17" )
+endif()
 set( CMAKE_CXX_STANDARD_REQUIRED ON )
 set( CMAKE_CXX_EXTENSIONS OFF )