Installing Neovim in Debian testing. This follows the rules for Ubuntu/Debian suggested in Neovim wiki

InstallNeovimNightly.bash

# As 17 July 2018 in Debian Testing upgraded 3 days ago.
# As root install the following:
# apt-get update
# apt-get install libtool libtool-bin autoconf automake cmake g++ pkg-config unzip gettext

# # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # 
# Get inside the neovim-nightly directory and let the fun begin
# # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # 

# As regular user then run this to build the third-party dependencies.
mkdir .deps # For building third-party dependencies
cd .deps
cmake ../third-party/
make -j 4

# The following will actually install Neovim
cd ..
mkdir build
cd build
cmake -DCMAKE_BUILD_TYPE=RelWithDebInfo .. # Release version with Debug info when using nightly builds as I do.
make -j 4
# as root
pip3 install pynvim # To add Python3 support
make install