* configure.ac: Add libelf to host_libs. Enable in-tree configury

* configure.ac: Add libelf to host_libs.  Enable in-tree configury
of ppl and cloog.  Fix in-tree configury of libelf, skip tests.
Fix portability of test of C++ as bootstrap language.  Add
ppl/src/ppl-config.o to the bootstrap compare exclusion list.
* configure: Rebuild.
* Makefile.def: Drop host and target settings from gmp, mpfr, ppl,
and cloog.  Fix in-tree ppl configuration.  Introduce libelf
in-tree building.
* Makefile.tpl (POSTSTAGE1_CXX_EXPORT): New.
(POSTSTAGE1_HOST_EXPORTS): Use it.
(STAGE[+id+]_CXXFLAGS): New.
(BASE_FLAGS_TO_PASS): Pass it down.
(configure-stage[+id+]-[+prefix+][+module+]): Use it.  Add
extra_exports.
(all-stage[+id+]-[+prefix+][+module+]): Likewise.
(configure-[+prefix+][+module+], all-[+prefix+][+module+]): Add
extra_exports.
* Makefile.in: Rebuild.
* configure.ac (FLAGS_FOR_TARGET):  Add -L and -isystem
paths for *-w64-mingw* and x86_64-*mingw*.
* configure: Regenerated.
* configure.ac: Disable target-winsup & co for
x86_64-*-mingw* and *-w64-mingw* targets.
* configure: Regenerated.
* configure.ac (CLooG test): Use = with test.
* configure: Regenerate.
* configure.ac: Do not set LIBS for ppl/cloog checks.  Disable
cloog if the ppl version check failed.  Move flags saving
before setting in libelf check.
* configure: Regenerate.
* configure.ac: Adjust the ppl and cloog configure to work as
documented.  Disable cloog if ppl was disabled.  Omit the version
checks if they were disabled.
* configure: Re-generate.