export: Factor out CMake-specific export generation (1/2)

In order to support generation of Common Package Specifications, the
mechanisms CMake uses to export package information need to be made more
abstract. As a first step toward this, refactor cmInstallExportGenerator
so that logic specific to config.cmake and Android .mk lives in separate
subclasses.

While we're at it, clean up the code style a bit and try to use moves a
bit more consistently.

This is step 1 of 2. The next step will refactor the individual file
generators along similar lines, which will also involve creating
additional classes for format-agnostic logic that is shared between
build-tree and install-tree variants.

Source/cmInstallAndroidMKExportGenerator.h

@@ -0,0 +1,40 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+#pragma once
+
+#include <iosfwd>
+#include <string>
+#include <vector>
+
+#include "cmInstallExportGenerator.h"
+
+class cmExportSet;
+class cmListFileBacktrace;
+
+/** \class cmInstallAndroidMKExportGenerator
+ * \brief Generate rules for creating Android .mk export files.
+ */
+class cmInstallAndroidMKExportGenerator : public cmInstallExportGenerator
+{
+public:
+  cmInstallAndroidMKExportGenerator(
+    cmExportSet* exportSet, std::string destination,
+    std::string filePermissions,
+    std::vector<std::string> const& configurations, std::string component,
+    MessageLevel message, bool excludeFromAll, std::string filename,
+    std::string targetNamespace, bool exportOld,
+    cmListFileBacktrace backtrace);
+  cmInstallAndroidMKExportGenerator(cmInstallAndroidMKExportGenerator const&) =
+    delete;
+  ~cmInstallAndroidMKExportGenerator() override;
+
+  cmInstallAndroidMKExportGenerator& operator=(
+    cmInstallAndroidMKExportGenerator const&) = delete;
+
+protected:
+  char const* InstallSubcommand() const override { return "EXPORT"; }
+
+  void GenerateScript(std::ostream& os) override;
+
+  bool const ExportOld;
+};