cmake: Add TCMALLOC_ENABLE_INSTALL with export/config infrastructure

Add a complete install and export system, mirroring abseil-cpp's
ABSL_ENABLE_INSTALL mechanism. Key changes:

- Add option(TCMALLOC_ENABLE_INSTALL): defaults to OFF when used as a
  subproject, ON when standalone (exactly matching Abseil's behavior)
- Remove the hard-coded CMAKE_SKIP_INSTALL_RULES=ON
- Create CMake/tcmallocConfig.cmake.in template (analogous to
  CMake/abslConfig.cmake.in) that finds Threads and absl dependencies
  then includes the exported targets file
- Add install(EXPORT tcmallocTargets) with tcmalloc:: namespace
- Add configure_package_config_file() to generate tcmallocConfig.cmake
- Add install(DIRECTORY tcmalloc ...) for public headers, excluding
  test-only headers (testing/, mock_*)

After this change, downstream projects can use:
  find_package(tcmalloc REQUIRED)
  target_link_libraries(myapp PRIVATE tcmalloc::tcmalloc)

The install rules in tcmalloc_cc_library() (added in the previous
commit) are automatically activated when TCMALLOC_ENABLE_INSTALL is ON.

Author: gtkramer

CMake/tcmallocConfig.cmake.in

@@ -0,0 +1,9 @@
+# tcmalloc CMake configuration file.
+
+include(CMakeFindDependencyMacro)
+find_dependency(Threads)
+find_dependency(absl)
+
+@PACKAGE_INIT@
+
+include("${CMAKE_CURRENT_LIST_DIR}/@PROJECT_NAME@Targets.cmake")

CMakeLists.txt

@@ -28,7 +28,14 @@ set(CMAKE_INSTALL_RPATH "$ORIGIN")
 set(CMAKE_INSTALL_RPATH_USE_LINK_PATH ON)
 set(CMAKE_BUILD_RPATH_USE_ORIGIN ON)
 
-set(CMAKE_SKIP_INSTALL_RULES ON)
+# When tcmalloc is included as a subproject (i.e. using
+# add_subdirectory(tcmalloc)), install rules are disabled by default.
+if(NOT CMAKE_SOURCE_DIR STREQUAL PROJECT_SOURCE_DIR)
+  option(TCMALLOC_ENABLE_INSTALL "Enable install rule" OFF)
+else()
+  option(TCMALLOC_ENABLE_INSTALL "Enable install rule" ON)
+endif()
+
 set(ABSL_PROPAGATE_CXX_STD ON)
 set(protobuf_ABSL_PROVIDER "package" CACHE STRING "" FORCE)
 set(protobuf_INSTALL OFF CACHE BOOL "" FORCE)
@@ -201,3 +208,31 @@ include(TcmallocVariants)
 add_subdirectory(tcmalloc/internal)
 add_subdirectory(tcmalloc/testing)
 add_subdirectory(tcmalloc)
+
+if(TCMALLOC_ENABLE_INSTALL)
+  # Export targets with tcmalloc:: namespace
+  install(EXPORT ${PROJECT_NAME}Targets
+    NAMESPACE tcmalloc::
+    DESTINATION "${CMAKE_INSTALL_LIBDIR}/cmake/${PROJECT_NAME}"
+  )
+
+  # Generate and install the config file from template
+  configure_package_config_file(
+    CMake/tcmallocConfig.cmake.in
+    "${PROJECT_BINARY_DIR}/${PROJECT_NAME}Config.cmake"
+    INSTALL_DESTINATION "${CMAKE_INSTALL_LIBDIR}/cmake/${PROJECT_NAME}"
+  )
+  install(FILES "${PROJECT_BINARY_DIR}/${PROJECT_NAME}Config.cmake"
+    DESTINATION "${CMAKE_INSTALL_LIBDIR}/cmake/${PROJECT_NAME}"
+  )
+
+  # Install public headers
+  install(DIRECTORY tcmalloc
+    DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
+    FILES_MATCHING
+      PATTERN "*.h"
+      PATTERN "*.inc"
+      PATTERN "testing" EXCLUDE
+      PATTERN "mock_*" EXCLUDE
+  )
+endif()