rapidhash: Make vendored header available to CMake sources

Source/Modules/CMakeBuildUtilities.cmake

@@ -102,6 +102,12 @@ else()
   CMAKE_SET_TARGET_FOLDER(cmlibrhash "Utilities/3rdParty")
 endif()
 
+#---------------------------------------------------------------------
+# rapidhash is a header-only hashing library used to speed up
+# string-keyed hash tables (e.g. cmDefinitions).  No system option is
+# provided because it has no conventional system package.
+add_subdirectory(Utilities/cmrapidhash)
+
 #---------------------------------------------------------------------
 # Build zlib library for Curl, CMake, and CTest.
 if(CMAKE_USE_SYSTEM_ZLIB)

Utilities/cm3p/rapidhash.h

@@ -0,0 +1,6 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file LICENSE.rst or https://cmake.org/licensing for details.  */
+#pragma once
+
+/* Use the rapidhash library bundled with CMake.  */
+#include <cmrapidhash/rapidhash.h> // IWYU pragma: export

Utilities/cmrapidhash/CMakeLists.txt

@@ -0,0 +1,6 @@
+# Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+# file LICENSE.rst or https://cmake.org/licensing for details.
+
+# rapidhash is a header-only library, so there is no library target to build.
+
+install(FILES LICENSE DESTINATION ${CMake_INSTALL_DOC_DIR}/cmrapidhash)