USE_GNUPG_ICONV.
(AC_CANONICAL_TARGET): Replaced by AC_CANONICAL_HOST. Changed all
reference to TARGET to HOST. Made sure that i586-mingw32msvc gets
detected as mingw32.
(UNIT64_C): Don't include inttype.h uncoditional but use a test to
ensure that it gets into conftest.h.
(AH_TOP): New. Prevent double inclusion of config.h.
* config.links: Use HOST instead of TARGET.
