Add property to mark IMPORTED targets as not SYSTEM

Add an `IMPORTED_NO_SYSTEM` target property to specify this.  When
enabled, do not treat the `INTERFACE_INCLUDE_DIRECTORIES` of an imported
target as `SYSTEM` include directories.

This is similar to the existing `NO_SYSTEM_FROM_IMPORTED` property, but
works from the consumed target rather than the consumer.

Fixes: #17364

Help/manual/cmake-properties.7.rst

@@ -239,6 +239,7 @@ Properties on Targets
    /prop_tgt/IMPORTED_LOCATION_CONFIG
    /prop_tgt/IMPORTED_NO_SONAME
    /prop_tgt/IMPORTED_NO_SONAME_CONFIG
+   /prop_tgt/IMPORTED_NO_SYSTEM
    /prop_tgt/IMPORTED_OBJECTS
    /prop_tgt/IMPORTED_OBJECTS_CONFIG
    /prop_tgt/IMPORTED_SONAME

Help/prop_tgt/IMPORTED_NO_SYSTEM.rst

@@ -0,0 +1,15 @@
+IMPORTED_NO_SYSTEM
+------------------
+
+Specifies that an :ref:`Imported Target <Imported Targets>` is not
+a ``SYSTEM`` library.  This has the following effects:
+
+* Entries of :prop_tgt:`INTERFACE_INCLUDE_DIRECTORIES` are not treated
+  as ``SYSTEM`` include directories when compiling consumers, as they
+  would be by default.
+
+Ignored for non-imported targets.
+
+See the :prop_tgt:`NO_SYSTEM_FROM_IMPORTED` target property to set this
+behavior on the target consuming the include directories rather than
+providing them.

Help/prop_tgt/NO_SYSTEM_FROM_IMPORTED.rst

@@ -13,3 +13,6 @@ imported targets as system includes.
 This property is initialized by the value of the
 :variable:`CMAKE_NO_SYSTEM_FROM_IMPORTED` variable if it is set when a target
 is created.
+
+See the :prop_tgt:`IMPORTED_NO_SYSTEM` target property to set this behavior
+on the target providing the include directories rather than consuming them.

Help/release/dev/imported-no-system.rst

@@ -0,0 +1,7 @@
+imported-no-system
+------------------
+
+* The :prop_tgt:`IMPORTED_NO_SYSTEM` target property was added to
+  specify that an :ref:`Imported Target <Imported Targets>` should
+  not be treated as a system library (i.e. its include directories
+  are not automatically ``SYSTEM``).

Source/cmGeneratorTarget.cxx

@@ -747,6 +747,9 @@ void handleSystemIncludesDep(cmLocalGenerator* lg,
   if (!depTgt->IsImported() || excludeImported) {
     return;
   }
+  if (depTgt->GetPropertyAsBool("IMPORTED_NO_SYSTEM")) {
+    return;
+  }
 
   if (cmValue dirs = depTgt->GetProperty("INTERFACE_INCLUDE_DIRECTORIES")) {
     cmExpandList(cmGeneratorExpression::Evaluate(*dirs, lg, config, headTarget,

Tests/IncludeDirectories/CMakeLists.txt

@@ -115,6 +115,13 @@ add_library(ordertest ordertest.cpp)
 target_include_directories(ordertest SYSTEM PUBLIC SystemIncludeDirectories/systemlib)
 target_include_directories(ordertest PUBLIC SystemIncludeDirectories/userlib)
 
+add_library(ordertest2 ordertest.cpp)
+target_include_directories(ordertest2 SYSTEM PRIVATE SystemIncludeDirectories/systemlib)
+target_link_libraries(ordertest2 PRIVATE ordertest2_userlib)
+add_library(ordertest2_userlib INTERFACE IMPORTED)
+target_include_directories(ordertest2_userlib INTERFACE SystemIncludeDirectories/userlib)
+set_property(TARGET ordertest2_userlib PROPERTY IMPORTED_NO_SYSTEM 1)
+
 add_subdirectory(StandardIncludeDirectories)
 add_subdirectory(TargetIncludeDirectories)