Re-enable the old behavior of --host and --build.

Re-enable the old behavior of --host and --build.

* acgeneral.m4 (build_alias): Set to host_alias if --build is not
given but --host is, and enable cross-compile auto-detection.  If
both are, and are different, enable cross compilation.
(AC_CANONICAL_HOST): Adjust help message.
(cross_compiling): Enclose in quotes when testing.
* aclang.m4 (_AC_LANG_COMPILER_WORKS): If cross_compiling is
maybe, set it to yes or no depending on the result of the
execution test.
* doc/autoconf.texi: Document the change.
* doc/install.texi: Likewise.