Revert "Allow target_link_libraries with IMPORTED targets."

This reverts commit 9cfe4f1b76.

It turns out that correctly adding the content to
the IMPORTED_LINK_INTERFACE_LIBARIES_<CONFIG> of an upstream target
from the buildsystem of a downstream project is not simple.

If upstream had added the INTERFACE content, the config-specific
properties would  be determined by the DEBUG_CONFIGURATIONS of
upstream.

As downstream, we don't have any information about what
the DEBUG_CONFIGURATIONS of upstream were, so we can't determine
which configuration-specific properties to populate. The best we can do
is add it to all of them or add it to the ones downstream considers to
be DEBUG_CONFIGURATIONS, neither of which is a good solution.

So, removing the porcelain API for that is the best approach. A human
can still determine which properties to populate and use
the set_property API to populate the desired properies.

Another solution to this would be for upstream targets to publish
what they consider DEBUG_CONFIGURATIONS, but that can be added in
a future release.

Source/cmTargetLinkLibrariesCommand.cxx

@@ -33,10 +33,6 @@ bool cmTargetLinkLibrariesCommand
   this->Target =
     this->Makefile->GetCMakeInstance()
     ->GetGlobalGenerator()->FindTarget(0, args[0].c_str());
-  if(!this->Target)
-    {
-    this->Target = this->Makefile->FindTargetToUse(args[0].c_str());
-    }
   if(!this->Target)
     {
     cmake::MessageType t = cmake::FATAL_ERROR;  // fail by default
@@ -261,16 +257,6 @@ cmTargetLinkLibrariesCommand::HandleLibrary(const char* lib,
   // Handle normal case first.
   if(this->CurrentProcessingState != ProcessingLinkInterface)
     {
-    if (this->Target->IsImported())
-      {
-      cmOStringStream e;
-      e << "Imported targets may only be used with the "
-          "LINK_INTERFACE_LIBRARIES specifier to target_link_libraries.";
-      this->Makefile->IssueMessage(cmake::FATAL_ERROR, e.str());
-      return;
-      }
-
-
     this->Makefile
       ->AddLinkLibraryForTarget(this->Target->GetName(), lib, llt);
     if (this->CurrentProcessingState != ProcessingPublicInterface)

Tests/CMakeCommands/target_link_libraries/CMakeLists.txt

@@ -29,9 +29,6 @@ set(CMAKE_INCLUDE_CURRENT_DIR ON)
 add_library(depA SHARED depA.cpp)
 generate_export_header(depA)
 
-add_library(importedlib UNKNOWN IMPORTED)
-target_link_libraries(importedlib LINK_INTERFACE_LIBRARIES depA)
-
 add_library(depB SHARED depB.cpp)
 generate_export_header(depB)