The 'build' script is documented as a way to build GNU Boot, and has been kept for now for compatibility reasons and/or for consistency with the rest of the documentation.

It has that:

# Some scripts like the ones in resources/packages/i945-thinkpads-install-utilities need
# Makefiles to be generated
. resources/scripts/misc/generate-configure-makefiles.sh

which has that:

if [ ! -f configure ] || \
       [ ! -f Makefile ] || \
       [ ! -f resources/packages/i945-thinkpads-install-utilities/Makefile ] ; then
    ./autogen.sh
    ./configure
fi

But if /dev/kvm is not usable ./configure will fail and require the user to use --disable-kvm as I thought it was better to make it fail like that instead of not failing and silently making the build potentially very slow.