Merge bitcoin/bitcoin#30584: depends: Make default host and build comparable

b28917be36 depends: Make default `host` and `build` comparable (Hennadii Stepanov)

Pull request description:

  To detect cross-compiling, the host and build platforms are compared. The `build` variable is always an output of `config.sub`, but the `host` is not. This can lead to false results. For example, on OpenBSD:
   - host=amd64-unknown-openbsd7.5
   - build=x86_64-unknown-openbsd7.5

  This PR sets the default value of the `host` variable to the value of `build`, ensuring cross-compiling won't be triggered when the `HOST` variable is not set.

  This PR fixes needless triggering of cross-compiling for CMake-built packages in depends on OpenBSD due to this code:eb85cacd29/depends/funcs.mk (L193-L197)

  No changes in Guix build.

ACKs for top commit:
  laanwj:
    Concept and code review ACK b28917be36
  theuni:
    utACK b28917be36.

Tree-SHA512: 8c5835cb8b739355b71f7cb161b350ad8b038a00e6b1def36354ba228cea3dcb9883df3c9a8e79d7d0143241f6f054129fe90772b1b2579702db51237f9d66ff

depends/Makefile

@@ -50,7 +50,6 @@ C_STANDARD ?= c11
 CXX_STANDARD ?= c++20
 
 BUILD = $(shell ./config.guess)
-HOST ?= $(BUILD)
 PATCHES_PATH = $(BASEDIR)/patches
 BASEDIR = $(CURDIR)
 HASH_LENGTH:=11
@@ -59,11 +58,6 @@ DOWNLOAD_RETRIES:=3
 HOST_ID_SALT ?= salt
 BUILD_ID_SALT ?= salt
 
-host:=$(BUILD)
-ifneq ($(HOST),)
-host:=$(HOST)
-endif
-
 ifneq ($(DEBUG),)
 release_type=debug
 else
@@ -73,9 +67,15 @@ endif
 base_build_dir=$(WORK_PATH)/build
 base_staging_dir=$(WORK_PATH)/staging
 base_download_dir=$(WORK_PATH)/download
-canonical_host:=$(shell ./config.sub $(HOST))
 build:=$(shell ./config.sub $(BUILD))
 
+host:=$(build)
+ifneq ($(HOST),)
+host:=$(HOST)
+endif
+HOST ?= $(BUILD)
+canonical_host:=$(shell ./config.sub $(HOST))
+
 build_arch =$(firstword $(subst -, ,$(build)))
 build_vendor=$(word 2,$(subst -, ,$(build)))
 full_build_os:=$(subst $(build_arch)-$(build_vendor)-,,$(build))