cmake: populate CMAKE_FIND_ROOT_PATH when cross-compiling for a different architecture on Linux

Author: illwieckz

CMakeLists.txt

@@ -416,6 +416,30 @@ set(COMMONLIST ${COMMONLIST} ${TINYFORMATLIST})
 # Libraries
 ################################################################################
 
+# Find libraries when cross-compiling for a different architecture on Linux systems.
+# We purposedly do not use the `LINUX` variable for two reasons:
+# - We're not testing for the target but for the system running the compiler,
+# - It should exclude Android.
+# Conveniently it also finds the MinGW root path when cross-compiling for Windows on Linux.
+if (CMAKE_SYSTEM_NAME MATCHES "Linux")
+    # This code setting MACHINE_TRIPLE may be moved to DaemonCompiler.
+    execute_process(COMMAND "${CMAKE_CXX_COMPILER}" -dumpmachine
+        OUTPUT_VARIABLE MACHINE_TRIPLE
+        OUTPUT_STRIP_TRAILING_WHITESPACE
+    )
+
+    # If the target architecture has a cross-compiler subfolder, use it.
+    if (MACHINE_TRIPLE)
+        set(MACHINE_ROOT_PATH "/usr/${MACHINE_TRIPLE}")
+
+        if (NOT IS_DIRECTORY "${MACHINE_ROOT_PATH}")
+            unset(MACHINE_ROOT_PATH)
+        endif()
+    endif()
+endif()
+
+list(APPEND CMAKE_FIND_ROOT_PATH ${MACHINE_ROOT_PATH})
+
 # Import external dependencies
 if (DEPS_DIR)
     # Warn on old version of deps
@@ -484,6 +508,7 @@ if (DEPS_DIR)
     set(CMAKE_INCLUDE_PATH ${DEPS_DIR} ${DEPS_DIR}/include ${CMAKE_INCLUDE_PATH})
     set(CMAKE_FRAMEWORK_PATH ${DEPS_DIR} ${CMAKE_FRAMEWORK_PATH})
     set(CMAKE_PREFIX_PATH ${DEPS_DIR} ${CMAKE_PREFIX_PATH})
+
     if (DAEMON_PARENT_SCOPE_DIR)
         # Also set parent scope so the top level CMakeLists can find precompiled deps
         set(CMAKE_FIND_ROOT_PATH ${DEPS_DIR} ${CMAKE_FIND_ROOT_PATH} PARENT_SCOPE)