flatland fails to build in ROS Noetic

[Educorreia932]

Trying to use ros-noetic-flatland package results in the following error:

error: builder for '/nix/store/qmsv1gf5ahjrgliwljjg8cdb6074bdn5-ros-noetic-flatland-msgs-1.3.3-r1.drv' failed with exit code 1;
       last 10 log lines:
       > -- Using these message generators: gencpp;geneus;genlisp;gennodejs;genpy
       > CMake Error at /nix/store/wjbqjwmnbi24qsvfh589my84fjr8wa3l-cmake-3.26.4/share/cmake-3.26/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
       >   Could NOT find PkgConfig (missing: PKG_CONFIG_EXECUTABLE)
       > Call Stack (most recent call first):
       >   /nix/store/wjbqjwmnbi24qsvfh589my84fjr8wa3l-cmake-3.26.4/share/cmake-3.26/Modules/FindPackageHandleStandardArgs.cmake:600 (_FPHSA_FAILURE_MESSAGE)
       >   /nix/store/wjbqjwmnbi24qsvfh589my84fjr8wa3l-cmake-3.26.4/share/cmake-3.26/Modules/FindPkgConfig.cmake:99 (find_package_handle_standard_args)
       >   CMakeLists.txt:17 (find_package)

[Educorreia932]

I have managed to fix the problem with flatland-msgs by adding pkg-config to its nativeBuildInputs, but now there are some problems with flatland-server related with the linkage of the Lua library:

error: builder for '/nix/store/7calkac1wqyph5dfz9gmsh5rdglccp8b-ros-noetic-flatland-server-1.3.3-r1.drv' failed with exit code 2;
       last 10 log lines:
       
> /nix/store/74y3751gsixaz9797ib0hp7c658sp1y5-binutils-2.40/bin/ld: devel/lib/libflatland_lib.so: undefined reference to `luaL_openlibs(lua_State*)'
> /nix/store/74y3751gsixaz9797ib0hp7c658sp1y5-binutils-2.40/bin/ld: devel/lib/libflatland_lib.so: undefined reference to `lua_pushboolean(lua_State*, int)'
> /nix/store/74y3751gsixaz9797ib0hp7c658sp1y5-binutils-2.40/bin/ld: devel/lib/libflatland_lib.so: undefined reference to `lua_pcallk(lua_State*, int, int, int, int, int (*)(lua_State*))'
> /nix/store/74y3751gsixaz9797ib0hp7c658sp1y5-binutils-2.40/bin/ld: devel/lib/libflatland_lib.so: undefined reference to `lua_isstring(lua_State*, int)'
> /nix/store/74y3751gsixaz9797ib0hp7c658sp1y5-binutils-2.40/bin/ld: devel/lib/libflatland_lib.so: undefined reference to `lua_pushstring(lua_State*, char const*)'
> /nix/store/74y3751gsixaz9797ib0hp7c658sp1y5-binutils-2.40/bin/ld: devel/lib/libflatland_lib.so: undefined reference to `lua_settop(lua_State*, int)'

[Educorreia932]

In the end, this was due to the fact that Lua was compiled to C, instead of C++.
To fix this, without having to recompile Lua, it's possible to encapsulate the Lua related #include statements (present in yaml_preprocessor.h file) with extern "C", as in this Stack Overflow answer.

[Educorreia932]

After making all the necessary changes, I've managed to successfully launch flatland_server #320