Update snapshot build to use Ubuntu 18.04 LTS

Ubuntu 16.04 would provide better distro compatibility, but we need SDL
in version 2.0.5 or newer to enable resizable window support on Linux.

Pretty soon Ubuntu 20.04 LTS will be released, so targetting previous
LTS release should be enough.

.github/workflows/linux.yml

@@ -64,13 +64,13 @@ jobs:
 
   build_linux_release_dynamic:
     name: Release build (dynamic)
-    runs-on: ubuntu-16.04
+    runs-on: ubuntu-18.04
     if: github.event_name != 'pull_request' || contains('dreamer,kcgen,ant-222', github.actor) == false
     steps:
       - uses: actions/checkout@v1
       - run:  sudo apt-get update
       - name: Install C++ compiler and libraries
-        run:  sudo apt-get install -y tree libpng16-dev librsvg2-bin $(./scripts/list-build-dependencies.sh -m apt -c gcc)
+        run:  sudo apt-get install -y tree libpng-dev librsvg2-bin $(./scripts/list-build-dependencies.sh -m apt -c gcc)
       - name:  Prepare compiler cache
         id:    prep-ccache
         shell: bash
@@ -109,7 +109,10 @@ jobs:
         run: |
           set -x
           ./autogen.sh
-          ./configure CFLAGS="$FLAGS" CXXFLAGS="$FLAGS" LDFLAGS="$FLAGS $LINKFLAGS -flto=$(nproc)" --disable-screenshots
+          ./configure \
+              CFLAGS="$FLAGS" CXXFLAGS="$FLAGS" \
+              LDFLAGS="$FLAGS $LINKFLAGS -flto=$(nproc)" \
+              --enable-png-static
           make -j "$(nproc)"
           strip src/dosbox
       - name: Package