cmake: Add cross-compiling support

To configure CMake for cross-compiling, use
`--toolchain depends/${HOST}/share/toolchain.cmake` command-line option.

CMakeLists.txt

@@ -88,6 +88,10 @@ if(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
   add_compile_definitions(MAC_OSX)
 endif()
 
+if(CMAKE_CROSSCOMPILING AND DEPENDS_ALLOW_HOST_PACKAGES)
+  list(APPEND CMAKE_FIND_ROOT_PATH "${CMAKE_SYSTEM_PREFIX_PATH}")
+endif()
+
 include(AddThreadsIfNeeded)
 add_threads_if_needed()
 
@@ -130,6 +134,12 @@ message("=================")
 message("Executables:")
 message("  bitcoind ............................ ${BUILD_DAEMON}")
 message("")
+if(CMAKE_CROSSCOMPILING)
+  set(cross_status "TRUE, for ${CMAKE_SYSTEM_NAME}, ${CMAKE_SYSTEM_PROCESSOR}")
+else()
+  set(cross_status "FALSE")
+endif()
+message("Cross compiling ....................... ${cross_status}")
 get_directory_property(definitions COMPILE_DEFINITIONS)
 string(REPLACE ";" " " definitions "${definitions}")
 message("Preprocessor defined macros ........... ${definitions}")

cmake/module/AddLibeventIfNeeded.cmake

@@ -37,15 +37,15 @@ function(add_libevent_if_needed)
     return()
   endif()
 
-  find_package(PkgConfig)
-  pkg_check_modules(libevent REQUIRED libevent>=${libevent_minimum_version} IMPORTED_TARGET GLOBAL)
+  include(CrossPkgConfig)
+  cross_pkg_check_modules(libevent REQUIRED libevent>=${libevent_minimum_version} IMPORTED_TARGET GLOBAL)
   check_evhttp_connection_get_peer(PkgConfig::libevent)
   target_link_libraries(PkgConfig::libevent INTERFACE
     $<$<BOOL:${MINGW}>:iphlpapi;ssp;ws2_32>
   )
   add_library(libevent::libevent ALIAS PkgConfig::libevent)
 
   if(NOT WIN32)
-    pkg_check_modules(libevent_pthreads REQUIRED libevent_pthreads>=${libevent_minimum_version} IMPORTED_TARGET)
+    cross_pkg_check_modules(libevent_pthreads REQUIRED libevent_pthreads>=${libevent_minimum_version} IMPORTED_TARGET)
   endif()
 endfunction()

cmake/module/CrossPkgConfig.cmake

@@ -0,0 +1,19 @@
+# Copyright (c) 2023 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+find_package(PkgConfig REQUIRED)
+
+macro(cross_pkg_check_modules)
+  if(CMAKE_CROSSCOMPILING)
+    set(pkg_config_path_saved "$ENV{PKG_CONFIG_PATH}")
+    set(pkg_config_libdir_saved "$ENV{PKG_CONFIG_LIBDIR}")
+    set(ENV{PKG_CONFIG_PATH} ${PKG_CONFIG_PATH})
+    set(ENV{PKG_CONFIG_LIBDIR} ${PKG_CONFIG_LIBDIR})
+    pkg_check_modules(${ARGV})
+    set(ENV{PKG_CONFIG_PATH} ${pkg_config_path_saved})
+    set(ENV{PKG_CONFIG_LIBDIR} ${pkg_config_libdir_saved})
+  else()
+    pkg_check_modules(${ARGV})
+  endif()
+endmacro()