What differs in cross-compiling mode?

[Christian Rössel]

configure switches to cross-compiling mode if I call it with build !=
host like in

./configure --build=powerpc64-unknown-linux-gnu
--host=powerpc-bgp-linux-gnu CC=bgxlc

Can someone explain to me what implications that has for configure
checks and the build in general? I can see that configure says

checking for powerpc-bgp-linux-gnu-gcc... bgxlc

and also the checks for ar, strip, ranlib are prefixed with $host, as
well as several libtool checks. What else is done different? Are some
tests omitted because you can't access the host during configure time
(e.g. on Blue Gene/P you can't access the host/compute-nodes directly)?

On Blue Gene/P systems you usually want to use the IBM xlc compiler
instead of gcc. So you usually explicitly specify the cross-compiler via
CC=bgxlc. If I specify CC directly, is there still the need to specify
--build and --host? Does configure behaves differently when using gcc or
bgxlc?

Is there a way to switch to normal non-cross-compiling mode by issuing
some commands in configure.ac?

Thanks,
Christian