target_link_libraries: Allow linking to UNKNOWN IMPORTED GLOBAL libs

This combination was accidentally rejected.  Allow it and add a test.

Fixes: #17245

Source/cmTargetLinkLibrariesCommand.cxx

@@ -366,6 +366,7 @@ bool cmTargetLinkLibrariesCommand::HandleLibrary(const std::string& lib,
 
       if (tgt && (tgt->GetType() != cmStateEnums::STATIC_LIBRARY) &&
           (tgt->GetType() != cmStateEnums::SHARED_LIBRARY) &&
+          (tgt->GetType() != cmStateEnums::UNKNOWN_LIBRARY) &&
           (tgt->GetType() != cmStateEnums::INTERFACE_LIBRARY) &&
           !tgt->IsExecutableWithExports()) {
         std::ostringstream e;

Tests/RunCMake/target_link_libraries/RunCMakeTest.cmake

@@ -10,3 +10,4 @@ run_cmake(SubDirTarget)
 run_cmake(SharedDepNotTarget)
 run_cmake(StaticPrivateDepNotExported)
 run_cmake(StaticPrivateDepNotTarget)
+run_cmake(UNKNOWN-IMPORTED-GLOBAL)

Tests/RunCMake/target_link_libraries/UNKNOWN-IMPORTED-GLOBAL.cmake

@@ -0,0 +1,4 @@
+enable_language(C)
+add_library(UnknownImportedGlobal UNKNOWN IMPORTED GLOBAL)
+add_library(mylib empty.c)
+target_link_libraries(mylib UnknownImportedGlobal)