configure.info: Target libiberty

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Target libiberty

   The `libiberty' subdirectory is currently a special case, in that it
is the only directory which is built both using the host compiler and
using the target compiler.
   This is because the files in `libiberty' are used when building the
host tools, and they are also incorporated into the `libstdc++' target
library as support code.
   This duality does not pose any particular difficulties.  It means
that there are targets for both `all-libiberty' and
`all-target-libiberty'.
   In a native configuration, when target libraries are not built in a
subdirectory, the same objects are normally used as both the host build
and the target build.  This is normally OK, since libiberty contains
only C code, and in a native configuration the results of the host
compiler and the target compiler are normally interoperable.
   Irix 6 is again an exception here, since the SGI native compiler
defaults to using the `O32' ABI, and gcc defaults to using the `N32'
ABI.  On Irix 6, the target libraries are built in a subdirectory even
for a native configuration, avoiding this problem.
   There are currently no other libraries built for both the host and
the target, but there is no conceptual problem with adding more.