librhash: Build the library within CMake

Update `ustd.h` to include KWSys Large File Support configuration so
that consistent stream libraries are used (on AIX with XL).

Add a `cm_rhash.h` header to include the CMake-provided copy of the
`rhash.h` header from CMake sources.

Source/CMakeLists.txt

@@ -788,6 +788,7 @@ target_link_libraries(CMakeLib cmsys
   ${CMAKE_CURL_LIBRARIES}
   ${CMAKE_JSONCPP_LIBRARIES}
   ${CMAKE_LIBUV_LIBRARIES}
+  ${CMAKE_LIBRHASH_LIBRARIES}
   ${CMake_KWIML_LIBRARIES}
   )