Fix configure_file output path by adding CMAKE_BINARY_DIR

[benjacam]

Motivation

I am trying to build the aws-iot-device-client with BUILD_SDK=OFF and where the CMake build directory is not placed directly under the aws-iot-device-client. The build failed, reporting (for the googletest case):

CMake Error at aws-iot-device-client/CMakeLists.txt:264 (add_subdirectory):
  add_subdirectory given source "/home/x/test/build/googletest-src" which
  is not an existing directory.

The motivation for having the build directory in a non-standard location is to build aws-iot-device-client against a seperate checkout of the SDK along with other software that uses the SDK as part of a larger cross-compile build.

Modifications

Change summary

The solution is to add ${CMAKE_BINARY_DIR} to the output argument of the configure_file CMake function. With this, the CMakeLists.txt.gtest or CMakeLists.txt.awssdk file will correctly be copied to the build/binary directory and the build completes successfully.

Testing

Failure case:
Example top level CMakeLists.txt which is part of larger build:

cmake_minimum_required(VERSION 3.10)
project(my-aws-project)
add_subdirectory(aws-iot-device-sdk-cpp-v2)
add_subdirectory(aws-iot-device-client)

Test steps:

git clone https://github.com/awslabs/aws-iot-device-client.git
git clone https://github.com/aws/aws-iot-device-sdk-cpp-v2.git
pushd aws-iot-device-sdk-cpp-v2; git submodule update --init --recursive; popd
mkdir build
cd build
cmake ../ -DCMAKE_TOOLCHAIN_FILE=$TOOLCHAIN -DBUILD_SDK=OFF
#
This fails!

This is fixed with this pull request.

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