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