build: set --build when configuring packages in depends

After reading
https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.70/autoconf.html#Specifying-Target-Triplets,
my understanding is that this change should mostly be a no-op, as
--build defaults to the output of config.guess, however, this may be
slightly more correct
>  For historical reasons, whenever you specify --host, be sure to
> specify --build too; this will be fixed in the future.

and will quell some warnings in depends (bitcoin#16354). If anything, this
also explicitly enables cross-compilation mode when `--host` differs
from `--build`.

As for "fixed in the future", this is the case for Autoconf 2.70+.

Author: fanquake

depends/funcs.mk

@@ -140,7 +140,13 @@ $(1)_config_env+=CMAKE_MODULE_PATH=$($($(1)_type)_prefix)/lib/cmake
 $(1)_config_env+=PATH=$(build_prefix)/bin:$(PATH)
 $(1)_build_env+=PATH=$(build_prefix)/bin:$(PATH)
 $(1)_stage_env+=PATH=$(build_prefix)/bin:$(PATH)
-$(1)_autoconf=./configure --host=$($($(1)_type)_host) --prefix=$($($(1)_type)_prefix) $$($(1)_config_opts) CC="$$($(1)_cc)" CXX="$$($(1)_cxx)"
+
+# Setting a --build type that differs from --host will explicitly enable
+# cross-compilation mode. Note that --build defaults to the output of
+# config.guess, which is what we set it too here. This also quells autoconf
+# warnings, "If you wanted to set the --build type, don't use --host.",
+# when using versions older than 2.70.
+$(1)_autoconf=./configure --build=$(BUILD) --host=$($($(1)_type)_host) --prefix=$($($(1)_type)_prefix) $$($(1)_config_opts) CC="$$($(1)_cc)" CXX="$$($(1)_cxx)"
 ifneq ($($(1)_nm),)
 $(1)_autoconf += NM="$$($(1)_nm)"
 endif