Add CMake package config file

This change enables automatic detection and consumption of Mbed TLS
library targets from within other CMake projects. By generating an
`MbedTLSConfig.cmake` file, consuming projects receive a more complete
view of these targets, allowing them to be used as dependencies which
properly inherit the transitive dependencies of the libraries.

This is fairly fragile, as it seems Mbed TLS's libraries do not appear
to properly model their dependencies on other targets, including
third-party dependencies. It is, however, sufficient for building and
linking the compiled Mbed TLS libraries when there are no third-party
dependencies involved. Further work is needed for more complex
use-cases, but this will likely meet the needs of most projects.

Resolves #298. Probably useful for #2857.

Signed-off-by: Chris Kay <chris.kay@arm.com>

programs/test/cmake_package/.gitignore

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

programs/test/cmake_package/CMakeLists.txt

@@ -0,0 +1,36 @@
+cmake_minimum_required(VERSION 2.8.12)
+
+#
+# Simulate configuring and building Mbed TLS as the user might do it. We'll
+# skip installing it, and use the build directory directly instead.
+#
+
+set(MbedTLS_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../../..")
+set(MbedTLS_BINARY_DIR "${CMAKE_CURRENT_BINARY_DIR}/mbedtls")
+
+execute_process(
+    COMMAND "${CMAKE_COMMAND}"
+        "-H${MbedTLS_SOURCE_DIR}"
+        "-B${MbedTLS_BINARY_DIR}"
+        "-DENABLE_PROGRAMS=NO"
+        "-DENABLE_TESTING=NO")
+
+execute_process(
+    COMMAND "${CMAKE_COMMAND}"
+        --build "${MbedTLS_BINARY_DIR}")
+
+#
+# Locate the package.
+#
+
+set(MbedTLS_DIR "${MbedTLS_BINARY_DIR}/cmake")
+find_package(MbedTLS REQUIRED)
+
+#
+# At this point, the Mbed TLS targets should have been imported, and we can now
+# link to them from our own program.
+#
+
+add_executable(cmake_package cmake_package.c)
+target_link_libraries(cmake_package
+    MbedTLS::mbedcrypto MbedTLS::mbedtls MbedTLS::mbedx509)

programs/test/cmake_package/cmake_package.c

@@ -0,0 +1,53 @@
+/*
+ *  Simple program to test that Mbed TLS builds correctly as a CMake package.
+ *
+ *  Copyright The Mbed TLS Contributors
+ *  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.
+ */
+
+#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 );
+}

programs/test/cmake_package_install/.gitignore

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

programs/test/cmake_package_install/CMakeLists.txt

@@ -0,0 +1,39 @@
+cmake_minimum_required(VERSION 2.8.12)
+
+#
+# Simulate configuring and building Mbed TLS as the user might do it. We'll
+# install into a directory inside our own build directory.
+#
+
+set(MbedTLS_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../../..")
+set(MbedTLS_INSTALL_DIR "${CMAKE_CURRENT_BINARY_DIR}/mbedtls")
+set(MbedTLS_BINARY_DIR "${MbedTLS_INSTALL_DIR}${CMAKE_FILES_DIRECTORY}")
+
+execute_process(
+    COMMAND "${CMAKE_COMMAND}"
+        "-H${MbedTLS_SOURCE_DIR}"
+        "-B${MbedTLS_BINARY_DIR}"
+        "-DENABLE_PROGRAMS=NO"
+        "-DENABLE_TESTING=NO"
+        "-DCMAKE_INSTALL_PREFIX=${MbedTLS_INSTALL_DIR}")
+
+execute_process(
+    COMMAND "${CMAKE_COMMAND}"
+        --build "${MbedTLS_BINARY_DIR}"
+        --target install)
+
+#
+# Locate the package.
+#
+
+set(MbedTLS_DIR "${MbedTLS_INSTALL_DIR}/cmake")
+find_package(MbedTLS REQUIRED)
+
+#
+# At this point, the Mbed TLS targets should have been imported, and we can now
+# link to them from our own program.
+#
+
+add_executable(cmake_package_install cmake_package_install.c)
+target_link_libraries(cmake_package_install
+    MbedTLS::mbedcrypto MbedTLS::mbedtls MbedTLS::mbedx509)

programs/test/cmake_package_install/cmake_package_install.c

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+/*
+ * Simple program to test that Mbed TLS builds correctly as an installable CMake
+ * package.
+ *
+ *  Copyright The Mbed TLS Contributors
+ *  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.
+ */
+
+#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 );
+}