Update GHA config to use libstdc++-11 with clang-13 through 15.

clang-12 through 15 do not fully support C++23, so it is not compatible
with libstdc++-13 in this mode. GHA config is enabling C++23 only since
clang-13, so update clang-13 through 15 jobs to use libstdc++-11.

Author: Lastique

.github/workflows/ci.yml

@@ -108,6 +108,7 @@ jobs:
             cxxstd: "11,14,17,2a"
             os: ubuntu-20.04
             install: clang-11
+          # Note: clang-12 through 15 do not fully support C++23, so it is not compatible with libstdc++-13 in this mode
           - toolset: clang
             compiler: clang++-12
             cxxstd: "11,14,17,2a"
@@ -117,17 +118,26 @@ jobs:
             compiler: clang++-13
             cxxstd: "11,14,17,20,2b"
             os: ubuntu-22.04
-            install: clang-13
+            install:
+              - clang-13
+              - g++-11
+            gcc_toolchain: 11
           - toolset: clang
             compiler: clang++-14
             cxxstd: "11,14,17,20,2b"
             os: ubuntu-22.04
-            install: clang-14
+            install:
+              - clang-14
+              - g++-11
+            gcc_toolchain: 11
           - toolset: clang
             compiler: clang++-15
             cxxstd: "11,14,17,20,2b"
             os: ubuntu-22.04
-            install: clang-15
+            install:
+              - clang-15
+              - g++-11
+            gcc_toolchain: 11
           - toolset: clang
             compiler: clang++-16
             cxxstd: "11,14,17,20,2b"
@@ -164,7 +174,19 @@ jobs:
         if: matrix.install
         run: |
           sudo apt-get update
-          sudo apt-get -y install ${{matrix.install}}
+          sudo apt-get -y install ${{join(matrix.install, ' ')}}
+
+      - name: Setup GCC Toolchain
+        if: matrix.gcc_toolchain
+        run: |
+            GCC_TOOLCHAIN_ROOT="$HOME/gcc-toolchain"
+            echo "GCC_TOOLCHAIN_ROOT=\"$GCC_TOOLCHAIN_ROOT\"" >> $GITHUB_ENV
+            MULTIARCH_TRIPLET="$(dpkg-architecture -qDEB_HOST_MULTIARCH)"
+            mkdir -p "$GCC_TOOLCHAIN_ROOT"
+            ln -s /usr/include "$GCC_TOOLCHAIN_ROOT/include"
+            ln -s /usr/bin "$GCC_TOOLCHAIN_ROOT/bin"
+            mkdir -p "$GCC_TOOLCHAIN_ROOT/lib/gcc/$MULTIARCH_TRIPLET"
+            ln -s "/usr/lib/gcc/$MULTIARCH_TRIPLET/${{matrix.gcc_toolchain}}" "$GCC_TOOLCHAIN_ROOT/lib/gcc/$MULTIARCH_TRIPLET/${{matrix.gcc_toolchain}}"
 
       - name: Setup Boost
         run: |
@@ -191,7 +213,15 @@ jobs:
       - name: Create user-config.jam
         if: matrix.compiler
         run: |
-          echo "using ${{matrix.toolset}} : : ${{matrix.compiler}} ;" > ~/user-config.jam
+          if [ -n "${{matrix.compiler}}" -o -n "$GCC_TOOLCHAIN_ROOT" ]
+          then
+              echo -n "using ${{matrix.toolset}} : : ${{matrix.compiler}}" > ~/user-config.jam
+              if [ -n "$GCC_TOOLCHAIN_ROOT" ]
+              then
+                  echo -n " : <compileflags>\"--gcc-toolchain=$GCC_TOOLCHAIN_ROOT\" <linkflags>\"--gcc-toolchain=$GCC_TOOLCHAIN_ROOT\"" >> ~/user-config.jam
+              fi
+              echo " ;" >> ~/user-config.jam
+          fi
 
       - name: Run tests
         run: |