cmake: Add cross-compiling support

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

CMakeLists.txt

@@ -479,6 +479,19 @@ else()
   )
 endif()
 
+target_compile_definitions(core_interface INTERFACE ${DEPENDS_COMPILE_DEFINITIONS})
+target_compile_definitions(core_interface_relwithdebinfo INTERFACE ${DEPENDS_COMPILE_DEFINITIONS_RELWITHDEBINFO})
+target_compile_definitions(core_interface_debug INTERFACE ${DEPENDS_COMPILE_DEFINITIONS_DEBUG})
+
+# If the {CXX,LD}FLAGS environment variables are defined during building depends
+# and configuring this build system, their content might be duplicated.
+if(DEFINED ENV{CXXFLAGS})
+  deduplicate_flags(CMAKE_CXX_FLAGS)
+endif()
+if(DEFINED ENV{LDFLAGS})
+  deduplicate_flags(CMAKE_EXE_LINKER_FLAGS)
+endif()
+
 if(BUILD_TESTS)
   enable_testing()
 endif()
@@ -538,6 +551,12 @@ message("  test_bitcoin-qt ..................... ${BUILD_GUI_TESTS}")
 message("  bench_bitcoin ....................... ${BUILD_BENCH}")
 message("  fuzz binary ......................... ${BUILD_FUZZ_BINARY}")
 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}")
 message("C++ compiler .......................... ${CMAKE_CXX_COMPILER_ID} ${CMAKE_CXX_COMPILER_VERSION}, ${CMAKE_CXX_COMPILER}")
 include(FlagsSummary)
 flags_summary()