Figure out a way to infer `BUILD` instead of making users set it

[alexrp]

kruco/bld/toolchain.mk

Lines 11 to 25 in f25c824

	# This is needed to convince Autotools that, yes, we do actually want to build a
	# cross-compiler even though we are (probably) running on an x86_64 machine.
	# Without this, x86_64-pc-linux-gnu is detected as the build machine triple,
	# while x86_64-linux-gnu as the target triple gets normalized to that as well,
	# so they compare equal.
	#
	# The downside of this hack is that BUILD needs to be set manually by the user
	# if building on some other kind of machine.
	#
	# TODO: Figure out a better solution?
	ifndef BUILD
	BUILD := x86_64-cross-linux-gnu
	$(warning BUILD variable not set; defaulting to $(BUILD))
	endif