libcapsule's use with biarch containers like the Steam Runtime will
frequently make it necessary to compile it for both x86_64 and i386.

On recent Debian-derived OSs this is OK, because the toolchain is
provided as a complete set of cross-compiler-style prefixed tools like
i686-linux-gnu-gcc; but some OSs, like Arch Linux and very old versions
of Debian, rely on 'gcc -m32' for their biarch support. This makes it
very easy to do

    ./configure --build=x86_64-linux-gnu --host=i686-linux-gnu

and accidentally produce x86_64 binaries, because there is no
i686-linux-gnu-gcc. Give the user a hint towards the correct invocation
in this case, which is:

    ./configure --build=x86_64-linux-gnu --host=i686-linux-gnu CC='gcc -m32'

I've implemented this as a reusable macro, in case we want to add it to
other projects that are likely to be cross-compiled by inexperienced
cross-compiler users.
Signed-off-by: Simon McVittie <smcv@collabora.com>