Re-enable screenshots feature for snapshot builds

With SDL2 in place, the silent dependency on libpng12 on Ubuntu 16.04 is
now gone. Unfortunately, development packages for libpng16 are named
differently for Ubuntu 16.04 and 18.04, so we can't add new package to
the global list.

.github/workflows/linux.yml

@@ -48,7 +48,7 @@ jobs:
       - uses: actions/checkout@v1
       - run:  sudo apt-get update
       - name: Install C++ compiler and libraries
-        run:  sudo apt-get install -y tree $(./scripts/list-build-dependencies.sh -m apt -c gcc)
+        run:  sudo apt-get install -y tree libpng16-dev $(./scripts/list-build-dependencies.sh -m apt -c gcc)
       - name: Log environment
         run:  ./scripts/log-env.sh
       - name: Inject version string
@@ -63,10 +63,7 @@ jobs:
         run: |
           set -x
           ./autogen.sh
-          ./configure \
-              --disable-screenshots \
-              CFLAGS="$FLAGS" \
-              CXXFLAGS="$FLAGS"
+          ./configure CFLAGS="$FLAGS" CXXFLAGS="$FLAGS"
           make -j "$(nproc)"
           strip src/dosbox
       - name: Package

README.md

@@ -66,15 +66,15 @@ be necessary for specific snapshots (check included README file).
 
 #### Fedora
 
-    $ sudo dnf install SDL2 SDL2_net opusfile
+    $ sudo dnf install libpng SDL2 SDL2_net opusfile
 
 #### Debian, Ubuntu
 
-    $ sudo apt install libsdl2-2.0 libsdl2-net-2.0 opusfile
+    $ sudo apt install libpng16-16 libsdl2-2.0 libsdl2-net-2.0 libopusfile0
 
 #### Arch, Manjaro
 
-    $  sudo pacman -S sdl2 sdl2_net opusfile
+    $  sudo pacman -S libpng sdl2 sdl2_net opusfile
 
 
 ## [Windows](https://github.com/dreamer/dosbox-staging/actions?query=workflow%3A%22Windows+builds%22+is%3Asuccess)