Identify DLL platforms via CMAKE_IMPORT_LIBRARY_SUFFIX

Use `CMAKE_IMPORT_LIBRARY_SUFFIX` to identify platforms that have DLL
import libraries rather than memorizing a list of platform names.

Fixes: #16801

Source/cmGeneratorTarget.cxx

@@ -132,8 +132,8 @@ cmGeneratorTarget::cmGeneratorTarget(cmTarget* t, cmLocalGenerator* lg)
                                      this->SourceEntries, true);
 
   this->DLLPlatform =
-    (this->Makefile->IsOn("WIN32") || this->Makefile->IsOn("CYGWIN") ||
-     this->Makefile->IsOn("MINGW"));
+    strcmp(this->Makefile->GetSafeDefinition("CMAKE_IMPORT_LIBRARY_SUFFIX"),
+           "") != 0;
 
   this->PolicyMap = t->PolicyMap;
 }

Source/cmInstallCommand.cxx

@@ -5,6 +5,7 @@
 #include "cmsys/Glob.hxx"
 #include <sstream>
 #include <stddef.h>
+#include <string.h>
 #include <utility>
 
 #include "cmAlgorithms.h"
@@ -334,8 +335,8 @@ bool cmInstallCommand::HandleTargetsMode(std::vector<std::string> const& args)
 
   // Check whether this is a DLL platform.
   bool dll_platform =
-    (this->Makefile->IsOn("WIN32") || this->Makefile->IsOn("CYGWIN") ||
-     this->Makefile->IsOn("MINGW"));
+    strcmp(this->Makefile->GetSafeDefinition("CMAKE_IMPORT_LIBRARY_SUFFIX"),
+           "") != 0;
 
   for (std::string const& tgt : targetList.GetVector()) {
 

Source/cmTarget.cxx

@@ -196,8 +196,8 @@ cmTarget::cmTarget(std::string const& name, cmStateEnums::TargetType type,
 
   // Check whether this is a DLL platform.
   this->DLLPlatform =
-    (this->Makefile->IsOn("WIN32") || this->Makefile->IsOn("CYGWIN") ||
-     this->Makefile->IsOn("MINGW"));
+    strcmp(this->Makefile->GetSafeDefinition("CMAKE_IMPORT_LIBRARY_SUFFIX"),
+           "") != 0;
 
   // Check whether we are targeting an Android platform.
   this->IsAndroid =