CMake: Add a subdirectory build regression test

If we have a regression with the "build Mbed TLS as a subdirectory with
CMake" feature and fail to build, fail the test.

programs/test/cmake_subproject/.gitignore

@@ -0,0 +1,3 @@
+build
+Makefile
+cmake_subproject

programs/test/cmake_subproject/CMakeLists.txt

@@ -0,0 +1,19 @@
+cmake_minimum_required(VERSION 2.6)
+
+# We use the parent Mbed TLS directory as the MBEDTLS_DIR for this test. Other
+# projects that use Mbed TLS as a subproject are likely to add by their own
+# relative paths.
+set(MBEDTLS_DIR ../../../)
+
+# Add Mbed TLS as a subdirectory.
+add_subdirectory(${MBEDTLS_DIR} build)
+
+# Link against all the Mbed TLS libraries.
+set(libs
+    mbedtls
+    mbedcrypto
+    mbedx509
+)
+
+add_executable(cmake_subproject cmake_subproject.c)
+target_link_libraries(cmake_subproject ${libs})

programs/test/cmake_subproject/cmake_subproject.c

@@ -0,0 +1,56 @@
+/*
+ *  Simple program to test that CMake builds with Mbed TLS as a subdirectory
+ *  work correctly.
+ *
+ *  Copyright (C) 2006-2019, ARM Limited, All Rights Reserved
+ *  SPDX-License-Identifier: Apache-2.0
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License"); you may
+ *  not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ *  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ *
+ *  This file is part of mbed TLS (https://tls.mbed.org)
+ */
+
+#if !defined(MBEDTLS_CONFIG_FILE)
+#include "mbedtls/config.h"
+#else
+#include MBEDTLS_CONFIG_FILE
+#endif
+
+#if defined(MBEDTLS_PLATFORM_C)
+#include "mbedtls/platform.h"
+#else
+#include <stdio.h>
+#include <stdlib.h>
+#define mbedtls_fprintf         fprintf
+#define mbedtls_printf          printf
+#define mbedtls_exit            exit
+#define MBEDTLS_EXIT_SUCCESS    EXIT_SUCCESS
+#define MBEDTLS_EXIT_FAILURE    EXIT_FAILURE
+#endif /* MBEDTLS_PLATFORM_C */
+
+#include "mbedtls/version.h"
+
+/* The main reason to build this is for testing the CMake build, so the program
+ * doesn't need to do very much. It calls a single library function to ensure
+ * linkage works, but that is all. */
+int main()
+{
+    /* This version string is 18 bytes long, as advised by version.h. */
+    char version[18];
+
+    mbedtls_version_get_string_full( version );
+
+    mbedtls_printf( "Built against %s\n", version );
+
+    return( 0 );
+}

tests/scripts/all.sh

@@ -242,6 +242,11 @@ cleanup()
     git checkout -- Makefile library/Makefile programs/Makefile tests/Makefile
     cd ..
 
+    # Remove any artifacts from the component_test_cmake_as_subdirectory test.
+    rm -rf programs/test/cmake_subproject/build
+    rm -f programs/test/cmake_subproject/Makefile
+    rm -f programs/test/cmake_subproject/cmake_subproject
+
     if [ -f "$CONFIG_BAK" ]; then
         mv "$CONFIG_BAK" "$CONFIG_H"
     fi
@@ -1248,6 +1253,19 @@ component_test_cmake_out_of_source () {
     unset MBEDTLS_ROOT_DIR
 }
 
+component_test_cmake_as_subdirectory () {
+    msg "build: cmake 'as-subdirectory' build"
+    MBEDTLS_ROOT_DIR="$PWD"
+
+    cd programs/test/cmake_subproject
+    cmake .
+    make
+    if_build_succeeded ./cmake_subproject
+
+    cd "$MBEDTLS_ROOT_DIR"
+    unset MBEDTLS_ROOT_DIR
+}
+
 component_test_zeroize () {
     # Test that the function mbedtls_platform_zeroize() is not optimized away by
     # different combinations of compilers and optimization flags by using an