install: Allow installing targets created in another directory

Previously, `install(TARGETS)` would only accept targets created in the same
directory scope.  Relax this restriction by searching the global scope when
determining whether or not a target exists.

Fixes: #14444

Source/cmInstallCommand.cxx

@@ -362,7 +362,12 @@ bool cmInstallCommand::HandleTargetsMode(std::vector<std::string> const& args)
       return false;
     }
     // Lookup this target in the current directory.
-    if (cmTarget* target = this->Makefile->FindLocalNonAliasTarget(tgt)) {
+    cmTarget* target = this->Makefile->FindLocalNonAliasTarget(tgt);
+    if (!target) {
+      // If no local target has been found, find it in the global scope.
+      target = this->Makefile->GetGlobalGenerator()->FindTarget(tgt, true);
+    }
+    if (target) {
       // Found the target.  Check its type.
       if (target->GetType() != cmStateEnums::EXECUTABLE &&
           target->GetType() != cmStateEnums::STATIC_LIBRARY &&
@@ -381,8 +386,7 @@ bool cmInstallCommand::HandleTargetsMode(std::vector<std::string> const& args)
     } else {
       // Did not find the target.
       std::ostringstream e;
-      e << "TARGETS given target \"" << tgt
-        << "\" which does not exist in this directory.";
+      e << "TARGETS given target \"" << tgt << "\" which does not exist.";
       this->SetError(e.str());
       return false;
     }

Source/cmInstallTargetGenerator.cxx

@@ -440,7 +440,13 @@ std::string cmInstallTargetGenerator::GetInstallFilename(
 
 void cmInstallTargetGenerator::Compute(cmLocalGenerator* lg)
 {
+  // Lookup this target in the current directory.
   this->Target = lg->FindLocalNonAliasGeneratorTarget(this->TargetName);
+  if (!this->Target) {
+    // If no local target has been found, find it in the global scope.
+    this->Target =
+      lg->GetGlobalGenerator()->FindGeneratorTarget(this->TargetName);
+  }
 }
 
 void cmInstallTargetGenerator::AddTweak(std::ostream& os, Indent indent,