configure: do not detect incorrect cross-compiler

configure: do not detect incorrect cross-compiler

On Ubuntu 11.04, configuring with --build=x86_64-unknown-linux-gnu
--host=x86_64-linux-gnu finds a wrong cross-compiler:
checking for cross-compiler... x86_64-x86_64-pc-linux-gnu-gcc

This happens because of a dash vs underscore inconsistency in configure.in:
host=x86_64-pc-linux-gnu
host_cpu=x86_64
host_os=linux-gnu
which ${host_cpu}-${host_os}-gcc >/dev/null 2>&1 && echo ${host_cpu}-${host-os}-gcc

This bug has been introduced in the initial --with-host support from
2002, commit eb267ade29c9a49c07b1c33dc9bf7a6790217400

This configure command is about "cross-compiling for i586", where the
system compiler is used, which just -m32 additional options.
The --build value comes from config.guess.