Now that SLADE can use GTK3 for wxWidgets on Linux, it should be much easier to compile SLADE on an out-of-the-box Ubuntu 18.04 installation.

You need to install the following packages:

sudo apt install git cmake build-essential libsfml-dev libwxgtk3.0-gtk3-dev \
libwxgtk-media3.0-gtk3-dev libwxgtk-webview3.0-gtk3-dev pkg-config \
libgtk-3-dev libfluidsynth-dev libfreeimage-dev libftgl-dev libglew-dev \
libcurl4-openssl-dev

You can optionally install these packages: sudo apt install libbz2-dev

Then, once all of those packages are installed, clone the repo: git clone https://github.com/sirjuddington/SLADE.git

Optionally, check out the stable branch: git checkout stable

Then, go to the dist folder, set up the build with CMake, and you'll be able to start compiling SLADE:

cd SLADE/dist
cmake -DWX_GTK3=ON ..
make

The -DWX_GTK3=ON tells wxWidgets to use GTK3. If you're compiling SLADE on Ubuntu 16.04, you will want to use -DWX_GTK3=OFF.