Maybe partially fix issue #76 where CMAKE_CXX_STANDARD wasn't affecti…

…ng probes for <filesystem>.

CMakeLists.txt

@@ -156,15 +156,18 @@ if(NOT MSVC OR CMAKE_VERSION VERSION_GREATER 3.59)
     cxx_variable_templates
   )
 endif()
+set(check_cxx_source_linkage_flags)
 # If on VS2019 16.3 or later, or on Apple, we require C++ 17
 if((MSVC AND MSVC_VERSION VERSION_GREATER_EQUAL 1923) OR APPLE)
   all_compile_features(PUBLIC
     cxx_std_17
   )
-  if(MSVC)
-    set(check_cxx_source_linkage_flags /std:c++17)
-  else()
-    set(check_cxx_source_linkage_flags -std=c++17)
+  if(NOT CMAKE_CXX_STANDARD)
+    if(MSVC)
+      set(check_cxx_source_linkage_flags /std:c++17)
+    else()
+      set(check_cxx_source_linkage_flags -std=c++17)
+    endif()
   endif()
 endif()
 # Set the library dependencies this library has
@@ -184,6 +187,13 @@ function(check_cxx_source_linkage prog var)
   if(MSVC AND CMAKE_GENERATOR MATCHES "Ninja")
     set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} /EHsc")
   endif()
+  if(CMAKE_CXX_STANDARD)
+    if(MSVC)
+      set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} /std:c++${CMAKE_CXX_STANDARD}")
+    else()
+      set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -std=c++${CMAKE_CXX_STANDARD}")
+    endif()
+  endif()
   if(CMAKE_SYSTEM_PROCESSOR STREQUAL ${CMAKE_HOST_SYSTEM_PROCESSOR} AND NOT LLFIO_ASSUME_CROSS_COMPILING)
     check_cxx_source_runs("${prog}" ${var})
   else()