Cross compilation

[Akim Demaille]

Years ago, it was suggested to simplify the handling of cross-compilation,
and in particular to no longer require --build: --host alone is the sign that
we're cross compiling.

In Autoconf 2.60, there still remains FIXME about this in general.m4.

How about getting rid of all this now?

Also, the piece of code that tries to infer whether we're cross-compiling:

if test "x$host_alias" != x; then
 if test "x$build_alias" = x; then
   cross_compiling=maybe
   echo "$as_me: WARNING: If you wanted to set the --build type,
don't use --host.
   If a cross compiler is detected then cross compile mode will be used." >&2
 elif test "x$build_alias" != "x$host_alias"; then
   cross_compiling=yes
 fi
fi

is run before config.site is loaded, so one can't define
host_alias and build_alias in it (I wouldn't recommend that in general,
but I see no reason to prevent it).

Thanks

PS/ Please, keep me in CC