cmLinkItem: Simplify tracking of whether link interface is explicit

We now only need a boolean.

Source/cmGeneratorTarget.cxx

@@ -5219,7 +5219,7 @@ void cmGeneratorTarget::ComputeLinkInterface(
   const std::string& config, cmOptionalLinkInterface& iface,
   cmGeneratorTarget const* headTarget) const
 {
-  if (iface.ExplicitLibraries) {
+  if (iface.Explicit) {
     if (this->GetType() == cmStateEnums::SHARED_LIBRARY ||
         this->GetType() == cmStateEnums::STATIC_LIBRARY ||
         this->GetType() == cmStateEnums::INTERFACE_LIBRARY) {
@@ -5659,7 +5659,7 @@ void cmGeneratorTarget::ComputeLinkInterfaceLibraries(
     return;
   }
   iface.Exists = true;
-  iface.ExplicitLibraries = explicitLibraries;
+  iface.Explicit = explicitLibraries != nullptr;
 
   if (explicitLibraries) {
     // The interface libraries have been explicitly set.

Source/cmLinkItem.h

@@ -87,7 +87,7 @@ struct cmOptionalLinkInterface : public cmLinkInterface
   bool LibrariesDone = false;
   bool AllDone = false;
   bool Exists = false;
-  const char* ExplicitLibraries = nullptr;
+  bool Explicit = false;
 };
 
 struct cmHeadToLinkInterfaceMap