It is important to restrict PATH as described above. If you have Strawberry Perl and/or Mercurial installed, it will pick up gcc & openssl from the wrong places.
If you are an Arch Linux user, refrain from updating system with `pacman -Syu`. Always update runtime separately as described on the home page, otherwise you might end up with DLLs incompatibility problems.
flatpak --user install flathub org.kde.Sdk/x86_64/5.12 # required SDK version might be higher, check the manifest at https://github.com/flathub/website.i2pd.i2pd/blob/master/website.i2pd.i2pd.json