ci: add a Docker image to test out C++ modules with GCC

.gitlab/ci/docker/gcc_cxx_modules/Dockerfile

@@ -0,0 +1,9 @@
+FROM fedora:36
+MAINTAINER Ben Boeckel <ben.boeckel@kitware.com>
+
+# Install build dependencies for packages.
+COPY install_deps.sh /root/install_deps.sh
+RUN sh /root/install_deps.sh
+
+COPY install_gcc.sh /root/install_gcc.sh
+RUN sh /root/install_gcc.sh

.gitlab/ci/docker/gcc_cxx_modules/install_deps.sh

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+#!/bin/sh
+
+set -e
+
+dnf install -y --setopt=install_weak_deps=False \
+    gcc-c++ mpfr-devel libmpc-devel isl-devel flex bison file findutils diffutils
+dnf clean all

.gitlab/ci/docker/gcc_cxx_modules/install_gcc.sh

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+#!/bin/sh
+
+set -e
+
+readonly revision="p1689r5-cmake-ci-20220614" # 3075e510e3d29583f8886b95aff044c0474c84a5
+readonly tarball="https://github.com/mathstuf/gcc/archive/$revision.tar.gz"
+
+readonly workdir="$HOME/gcc"
+readonly srcdir="$workdir/gcc"
+readonly builddir="$workdir/build"
+readonly njobs="$( nproc )"
+
+mkdir -p "$workdir"
+cd "$workdir"
+curl -L "$tarball" > "gcc-$revision.tar.gz"
+tar xf "gcc-$revision.tar.gz"
+mv "gcc-$revision" "$srcdir"
+mkdir -p "$builddir"
+cd "$builddir"
+"$srcdir/configure" \
+    --disable-multilib \
+    --enable-languages=c,c++ \
+    --prefix="/opt/gcc-p1689"
+make "-j$njobs"
+make "-j$njobs" install-strip
+rm -rf "$workdir"