fix(build): fix passing CMAKE_<LANG>_FLAGS from ExternalProject_Add

tools/test_apps/build_system/external_project/CMakeLists.txt

@@ -0,0 +1,10 @@
+# The following lines of boilerplate have to be in your project's
+# CMakeLists in this exact order for cmake to work correctly
+cmake_minimum_required(VERSION 3.22)
+
+include($ENV{IDF_PATH}/tools/cmake/project.cmake)
+
+# Trim the build. Include the minimal set of components, main, and anything it depends on.
+set(COMPONENTS main)
+
+project(external_project)

tools/test_apps/build_system/external_project/README.md

@@ -0,0 +1,2 @@
+| Supported Targets | ESP32 | ESP32-C3 |
+| ----------------- | ----- | -------- |

tools/test_apps/build_system/external_project/components/ext_component/CMakeLists.txt

@@ -0,0 +1,19 @@
+idf_component_register()
+
+include(ExternalProject)
+externalproject_add(ext_subproject
+    SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/subproject
+    CMAKE_ARGS
+        -DCMAKE_TOOLCHAIN_FILE=${CMAKE_TOOLCHAIN_FILE}
+        -DCMAKE_C_FLAGS=-DEXTERNAL_PROJECT_FLAG=1
+        -DCMAKE_CXX_FLAGS=-DEXTERNAL_PROJECT_CXX_FLAG=1
+        -DCMAKE_ASM_FLAGS=-DEXTERNAL_PROJECT_ASM_FLAG=1
+        -DCMAKE_EXE_LINKER_FLAGS=-Wl,--defsym=EXTERNAL_PROJECT_LINKER_CHECK=1
+    BINARY_DIR "${CMAKE_CURRENT_BINARY_DIR}/subproject"
+    INSTALL_COMMAND ""
+    BUILD_BYPRODUCTS "${CMAKE_CURRENT_BINARY_DIR}/subproject/libext_lib.a"
+)
+
+add_prebuilt_library(ext_lib "${CMAKE_CURRENT_BINARY_DIR}/subproject/libext_lib.a")
+
+target_link_libraries(${COMPONENT_LIB} INTERFACE ext_lib)

tools/test_apps/build_system/external_project/components/ext_component/subproject/CMakeLists.txt

@@ -0,0 +1,6 @@
+cmake_minimum_required(VERSION 3.22)
+
+project(ext_subproject LANGUAGES C CXX ASM)
+add_library(ext_lib STATIC check_flag.c check_flag.cpp check_flag.S)
+add_executable(ext_link_test link_check.c)
+target_link_libraries(ext_link_test PRIVATE ext_lib)

tools/test_apps/build_system/external_project/components/ext_component/subproject/check_flag.S

@@ -0,0 +1,9 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Unlicense OR CC0-1.0
+ */
+
+#ifndef EXTERNAL_PROJECT_ASM_FLAG
+#error "EXTERNAL_PROJECT_ASM_FLAG not defined - toolchain did not preserve CMAKE_ASM_FLAGS from ExternalProject_Add"
+#endif

tools/test_apps/build_system/external_project/components/ext_component/subproject/check_flag.c

@@ -0,0 +1,9 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Unlicense OR CC0-1.0
+ */
+
+#ifndef EXTERNAL_PROJECT_FLAG
+#error "EXTERNAL_PROJECT_FLAG not defined - toolchain did not preserve CMAKE_C_FLAGS from ExternalProject_Add"
+#endif

tools/test_apps/build_system/external_project/components/ext_component/subproject/check_flag.cpp

@@ -0,0 +1,9 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Unlicense OR CC0-1.0
+ */
+
+#ifndef EXTERNAL_PROJECT_CXX_FLAG
+#error "EXTERNAL_PROJECT_CXX_FLAG not defined - toolchain did not preserve CMAKE_CXX_FLAGS from ExternalProject_Add"
+#endif

tools/test_apps/build_system/external_project/components/ext_component/subproject/link_check.c

@@ -0,0 +1,11 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Unlicense OR CC0-1.0
+ */
+extern char EXTERNAL_PROJECT_LINKER_CHECK;
+
+int _start(void)
+{
+    return (int)&EXTERNAL_PROJECT_LINKER_CHECK;
+}

tools/test_apps/build_system/external_project/main/CMakeLists.txt

@@ -0,0 +1,2 @@
+idf_component_register(SRCS "test_main.c"
+                       REQUIRES ext_component)

tools/test_apps/build_system/external_project/main/test_main.c

@@ -0,0 +1,12 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Unlicense OR CC0-1.0
+ */
+
+#include <stdio.h>
+
+void app_main(void)
+{
+    printf("external_project test OK\n");
+}