Avoid spurious libgcc-s1 Debian package dependency on Ubuntu 18.04

GitHub actions have GCC 10 available, which resulted in GCC libraries
being upgraded as well. Forcibly downgrade this to Bionic's (18.04)
native version, removing GCC 9 and 10 along the way.

Using `container: ubuntu:18.04` would potentially be a cleaner solution,
but our use of submodules requires Git >= 2.18, which isn't in Ubuntu
18.04. So we'd need to start mixing version along a different path.

Fixes: #5875

Author: tautschnig

.github/workflows/pull-request-checks.yaml

@@ -375,6 +375,10 @@ jobs:
         run: | 
           sudo apt-get update
           sudo apt-get install --no-install-recommends -y g++ flex bison cmake ninja-build maven jq libxml2-utils dpkg-dev ccache
+          # remove libgcc-s1, which isn't normally available in Ubuntu 18.04
+          target=$(dpkg-query -W --showformat='${Version}\n' gcc-8-base | head -n 1)
+          # libgcc1 uses an epoch, thus the extra 1:
+          sudo apt-get install -y --allow-downgrades --reinstall gcc g++ libgcc-s1- libstdc++6=$target liblsan0=$target libtsan0=$target libcc1-0=$target libgcc1=1:$target
       - name: Prepare ccache
         uses: actions/cache@v2
         with:

.github/workflows/release-packages.yaml

@@ -69,7 +69,12 @@ jobs:
         with:
           submodules: recursive
       - name: Fetch dependencies
-        run: sudo apt-get install --no-install-recommends -y g++ flex bison cmake ninja-build maven jq libxml2-utils dpkg-dev ccache
+        run: |
+          sudo apt-get install --no-install-recommends -y g++ flex bison cmake ninja-build maven jq libxml2-utils dpkg-dev ccache
+          # remove libgcc-s1, which isn't normally available in Ubuntu 18.04
+          target=$(dpkg-query -W --showformat='${Version}\n' gcc-8-base | head -n 1)
+          # libgcc1 uses an epoch, thus the extra 1:
+          sudo apt-get install -y --allow-downgrades --reinstall gcc g++ libgcc-s1- libstdc++6=$target liblsan0=$target libtsan0=$target libcc1-0=$target libgcc1=1:$target
       - name: Prepare ccache
         uses: actions/cache@v2
         with:

cmake/packaging.cmake

@@ -27,6 +27,10 @@ set(CPACK_DEBIAN_PACKAGE_SHLIBDEPS YES)
 # In addition, we depend on gcc for preprocessing
 set(CPACK_DEBIAN_PACKAGE_DEPENDS gcc)
 
+# Enable debug output so that we can see the dependencies being generated in the
+# logs
+set(CPACK_DEBIAN_PACKAGE_DEBUG YES)
+
 # For windows we need to set up product and update GUID
 # See: https://docs.microsoft.com/en-us/windows/win32/msi/productcode
 # and  https://docs.microsoft.com/en-us/windows/win32/msi/upgradecode