Make fails following building steps

[guilyx]

Hi,

The building.md files states the following:

git clone https://github.com/potassco/plasp.git
cd plasp
mkdir -p build/release
cd build/release
cmake ../.. -DCMAKE_BUILD_TYPE=Release
make

But this needs to be run for a make that doesn't fail:

git submodule init
git submodule update

Otherwise it fails stating:

 fatal error: mapbox/variant.hpp: No such file or directory
 #include <mapbox/variant.hpp>
          ^~~~~~~~~~~~~~~~~~~~

[ZimneJonas]

run in the cloned plasp/ directory:
$ git submodule update --init --recursive