[CMake] Use correct variable as header install prefix

[phosek]

Summary

This variable is already set in CMakeLists.txt but it wasn't used
which means that the headers get installed into a wrong location
when the per target runtime directory option is being used.

[smeenai]

Seems like an obvious enough fix, so I'm jumping in to LGTM.

[phosek]

Thank you!

libcxx/trunk/include/CMakeLists.txt

else()
   add_custom_target(cxx-headers)
 endif()
 set_target_properties(cxx-headers PROPERTIES FOLDER "Misc")
 
 if (LIBCXX_INSTALL_HEADERS)
   foreach(file ${files})
     get_filename_component(dir ${file} DIRECTORY)
     install(FILES ${file}
-      DESTINATION ${LIBCXX_INSTALL_PATH}include/c++/v1/${dir}
+      DESTINATION ${LIBCXX_INSTALL_HEADER_PREFIX}include/c++/v1/${dir}
       COMPONENT cxx-headers
       PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ
     )
   endforeach()
 
   if (LIBCXX_NEEDS_SITE_CONFIG)
     # Install the generated header as __config.
     install(FILES ${LIBCXX_BINARY_DIR}/__generated_config
-      DESTINATION ${LIBCXX_INSTALL_PATH}include/c++/v1
+      DESTINATION ${LIBCXX_INSTALL_HEADER_PREFIX}include/c++/v1
       PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ
       RENAME __config
       COMPONENT cxx-headers)
   endif()
 
   if (NOT CMAKE_CONFIGURATION_TYPES)
     add_custom_target(install-cxx-headers
                       DEPENDS cxx-headers ${generated_config_deps}