add_subdirectory: Run subdirectory install rules in correct order

Before this change, install rules created by add_subdirectory()
would be executed after all of the top-level install rules, even
if they were declared before the top-level rules. This change
adds a new policy, CMP0082, which interleaves the add_subdirectory()
install rules with the other install rules so they are run in the
correct order.

Source/cmInstallSubdirectoryGenerator.h

@@ -0,0 +1,41 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+#ifndef cmInstallSubdirectoryGenerator_h
+#define cmInstallSubdirectoryGenerator_h
+
+#include "cmConfigure.h" // IWYU pragma: keep
+
+#include "cmInstallGenerator.h"
+
+#include <iosfwd>
+#include <string>
+
+class cmLocalGenerator;
+class cmMakefile;
+
+/** \class cmInstallSubdirectoryGenerator
+ * \brief Generate target installation rules.
+ */
+class cmInstallSubdirectoryGenerator : public cmInstallGenerator
+{
+public:
+  cmInstallSubdirectoryGenerator(cmMakefile* makefile,
+                                 const char* binaryDirectory,
+                                 bool excludeFromAll);
+  ~cmInstallSubdirectoryGenerator() override;
+
+  bool HaveInstall() override;
+  void CheckCMP0082(bool& haveSubdirectoryInstall,
+                    bool& haveInstallAfterSubdirectory) override;
+
+  void Compute(cmLocalGenerator* lg) override;
+
+protected:
+  void GenerateScript(std::ostream& os) override;
+
+  cmMakefile* Makefile;
+  std::string BinaryDirectory;
+  cmLocalGenerator* LocalGenerator;
+};
+
+#endif