configure: install in a sub-dir of --libdir and --docdir

In case the user specifies the LIBDIR with --libdir , or DOCDIR with
--docdir, install our support files in a sub-dir, so as not to mangle
existing (potentially system) files/dirs.
(transplanted from 991fc21dd3cfda7d6448388b3224821f6937f15e)

 # ia64 specific config options

 # depends on EXPERIMENTAL

 config ARCH_ia64

     select ARCH_USE_MMU

     select ARCH_64

     select ARCH_SUPPORTS_BOTH_ENDIAN

     help

       The ia64 architecture, as defined by:

         http://www.intel.com/design/itanium/arch_spec.htm