ci: Travis: use minimum supported CMake in one job (#10445)

.travis.yml

@@ -80,6 +80,8 @@ jobs:
       compiler: gcc
       env:
         - BUILD_32BIT=ON
+        # Minimum required CMake.
+        - CMAKE_URL=https://cmake.org/files/v2.8/cmake-2.8.12-Linux-i386.sh
         - *common-job-env
     - if: branch = master AND commit_message !~ /\[skip.lint\]/
       name: lint

CMakeLists.txt

@@ -2,6 +2,7 @@
 #   intro: https://codingnest.com/basic-cmake/
 #   best practices (3.0+): https://gist.github.com/mbinna/c61dbb39bca0e4fb7d1f73b0d66a4fd1
 
+# Version should match the tested CMAKE_URL in .travis.yml.
 cmake_minimum_required(VERSION 2.8.12)
 project(nvim C)
 

ci/before_install.sh

@@ -46,3 +46,16 @@ fi
 source ~/.nvm/nvm.sh
 nvm install --lts
 nvm use --lts
+
+if [[ -n "$CMAKE_URL" ]]; then
+  echo "Installing custom CMake: $CMAKE_URL"
+  curl --retry 5 --silent --fail -o /tmp/cmake-installer.sh "$CMAKE_URL"
+  mkdir -p "$HOME/.local/bin" /opt/cmake-custom
+  bash /tmp/cmake-installer.sh --prefix=/opt/cmake-custom --skip-license
+  ln -sfn /opt/cmake-custom/bin/cmake "$HOME/.local/bin/cmake"
+  cmake_version="$(cmake --version)"
+  echo "$cmake_version" | grep -qF '2.8.12' || {
+    echo "Unexpected CMake version: $cmake_version"
+    exit 1
+  }
+fi