cross-compiling: handling modern platforms able to run foreign binaries.

* lib/autoconf/lang.m4 (_AC_COMPILER_EXEEXT_CROSS):
When only --host is set, and no --build, but in the end, the resulting
computed $build and $host are different, set $cross_compiling = yes.
Indeed we can't rely only on a successful test program run, because false
positives occur on some platforms. In particular modern GNU/Linux
distributions set Wine to automatically handle Windows binaries.
Consequently $cross_compiling gets set to "no" even though we are in an
obvious cross-compilation case.
(tiny change)