target_*: Allow setting INTERFACE properties of UNKNOWN IMPORTED targets

Extend the change made by commit fe4b25ec2f (Teach target_* commands to
set INTERFACE properties of IMPORTED targets, 2017-09-18,
v3.11.0-rc1~433^2~2) to work with imported targets of type `UNKNOWN`.

Fixes: #19434

Source/cmTargetPropCommandBase.cxx

@@ -37,7 +37,8 @@ bool cmTargetPropCommandBase::HandleArguments(
       (this->Target->GetType() != cmStateEnums::SHARED_LIBRARY) &&
       (this->Target->GetType() != cmStateEnums::MODULE_LIBRARY) &&
       (this->Target->GetType() != cmStateEnums::OBJECT_LIBRARY) &&
-      (this->Target->GetType() != cmStateEnums::INTERFACE_LIBRARY)) {
+      (this->Target->GetType() != cmStateEnums::INTERFACE_LIBRARY) &&
+      (this->Target->GetType() != cmStateEnums::UNKNOWN_LIBRARY)) {
     this->SetError("called with non-compilable target type");
     return false;
   }

Tests/RunCMake/add_library/UNKNOWNwithOnlyObjectSources-stderr.txt

@@ -1,4 +1,4 @@
 ^CMake Error at UNKNOWNwithOnlyObjectSources.cmake:[0-9]+ \(target_sources\):
-  target_sources called with non-compilable target type
+  target_sources may only set INTERFACE properties on IMPORTED targets
 Call Stack \(most recent call first\):
   CMakeLists.txt:[0-9]+ \(include\)$

Tests/RunCMake/target_compile_definitions/RunCMakeTest.cmake

@@ -1,3 +1,4 @@
 include(RunCMake)
 
 run_cmake(empty_keyword_args)
+run_cmake(unknown_imported_target)

Tests/RunCMake/target_compile_definitions/unknown_imported_target.cmake

@@ -0,0 +1,11 @@
+# Test that target_compile_definitions works on UNKNOWN IMPORTED target
+add_library(imported UNKNOWN IMPORTED)
+target_compile_definitions(imported INTERFACE FOO)
+
+get_target_property(IMPORTED_INTERFACE_CDS imported INTERFACE_COMPILE_DEFINITIONS)
+
+if (NOT FOO IN_LIST IMPORTED_INTERFACE_CDS)
+  message(
+    FATAL_ERROR "FOO should be in INTERFACE_COMPILE_DEFINITIONS.\n"
+    "Actual INTERFACE_COMPILE_DEFINITIONS: " ${IMPORTED_INTERFACE_CDS})
+endif()