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.cxx

@@ -0,0 +1,77 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+#include "cmInstallSubdirectoryGenerator.h"
+
+#include "cmLocalGenerator.h"
+#include "cmMakefile.h"
+#include "cmPolicies.h"
+#include "cmScriptGenerator.h"
+#include "cmSystemTools.h"
+
+#include <sstream>
+#include <vector>
+
+cmInstallSubdirectoryGenerator::cmInstallSubdirectoryGenerator(
+  cmMakefile* makefile, const char* binaryDirectory, bool excludeFromAll)
+  : cmInstallGenerator(nullptr, std::vector<std::string>(), nullptr,
+                       MessageDefault, excludeFromAll)
+  , Makefile(makefile)
+  , BinaryDirectory(binaryDirectory)
+{
+}
+
+cmInstallSubdirectoryGenerator::~cmInstallSubdirectoryGenerator()
+{
+}
+
+bool cmInstallSubdirectoryGenerator::HaveInstall()
+{
+  for (auto generator : this->Makefile->GetInstallGenerators()) {
+    if (generator->HaveInstall()) {
+      return true;
+    }
+  }
+
+  return false;
+}
+
+void cmInstallSubdirectoryGenerator::CheckCMP0082(
+  bool& haveSubdirectoryInstall, bool& /*unused*/)
+{
+  if (this->HaveInstall()) {
+    haveSubdirectoryInstall = true;
+  }
+}
+
+void cmInstallSubdirectoryGenerator::Compute(cmLocalGenerator* lg)
+{
+  this->LocalGenerator = lg;
+}
+
+void cmInstallSubdirectoryGenerator::GenerateScript(std::ostream& os)
+{
+  if (!this->ExcludeFromAll) {
+    cmPolicies::PolicyStatus status =
+      this->LocalGenerator->GetPolicyStatus(cmPolicies::CMP0082);
+    switch (status) {
+      case cmPolicies::WARN:
+      case cmPolicies::OLD:
+        // OLD behavior is handled in cmLocalGenerator::GenerateInstallRules()
+        break;
+
+      case cmPolicies::NEW:
+      case cmPolicies::REQUIRED_IF_USED:
+      case cmPolicies::REQUIRED_ALWAYS: {
+        Indent indent;
+        std::string odir = this->BinaryDirectory;
+        cmSystemTools::ConvertToUnixSlashes(odir);
+        os << indent << "if(NOT CMAKE_INSTALL_LOCAL_ONLY)\n"
+           << indent.Next()
+           << "# Include the install script for the subdirectory.\n"
+           << indent.Next() << "include(\"" << odir
+           << "/cmake_install.cmake\")\n"
+           << indent << "endif()\n\n";
+      } break;
+    }
+  }
+}