Add option to append MachineCodeComputer_SystemdDBusInodes_SHA256.cpp to list of source files

[nathan-hess]

This change solves an issue I ran into when attempting to use the Cryptolens C++ API with the MachineCodeComputer_SystemdDBusInodes_SHA256 machine code computer (on Linux).

Issue Description

The issue can be recreated as follows:

1. Create a GitHub Codespace instance in the Cryptolens/cryptolens-cpp repository
2. Replace examples/unix/example_activate.cpp with example_activate.zip
3. Run the following commands:

sudo apt update
sudo apt install ninja-build
mkdir examples/unix/cmake/build
cd examples/unix/cmake/build
cmake .. -G Ninja
ninja

A linker error similar to the following should be displayed:

/usr/bin/ld: CMakeFiles/example_activate.dir/workspaces/cryptolens-cpp/examples/unix/example_activate.cpp.o: in function `cryptolens_io::v20190401::basic_Cryptolens<cryptolens_io::v20190401::Configuration_Unix<cryptolens_io::v20190401::MachineCodeComputer_SystemdDBusInodes_SHA256> >::activate(cryptolens_io::v20190401::basic_Error&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, int, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, int)':
example_activate.cpp:([...]): undefined reference to `cryptolens_io::v20190401::MachineCodeComputer_SystemdDBusInodes_SHA256::get_machine_code[abi:cxx11](cryptolens_io::v20190401::basic_Error&)'
collect2: error: ld returned 1 exit status
ninja: build stopped: subcommand failed.

Proposed Solution

This change mirrors the same option already present to enable compiling with access to MachineCodeComputer_COM:

cryptolens-cpp/CMakeLists.txt

Lines 36 to 40 in 68a9dc2

	set (CRYPTOLENS_BUILD_MACHINE_CODE_COM OFF CACHE BOOL "build with MachineCodeComputer_COM?")
	if (CRYPTOLENS_BUILD_MACHINE_CODE_COM)
	list (APPEND SRC "src/MachineCodeComputer_COM.cpp" "third_party/curl/isunreserved.cpp")
	list (APPEND LIBS "Iphlpapi")
	endif()

With this change, the cache variable CRYPTOLENS_BUILD_MACHINE_CODE_SYSTEMDDBUSINODES can be set (for instance, in examples/unix/cmake/CMakeLists.txt) if using the MachineCodeComputer_SystemdDBusInodes_SHA256 machine code computer.

I'm not super familiar with compilers, but I suspect this works because if MachineCodeComputer_SystemdDBusInodes_SHA256.cpp is not explicitly specified, GCC still finds and compiles it but without the desired flags that CMake otherwise adds for options such as C++ standard.

[svedi]

The commit looks good! I will look into merging it in the coming days, thanks!

[nathan-hess]

The commit looks good! I will look into merging it in the coming days, thanks!

Excellent, thanks for looking it over!