ci: Use ccache to speed up repeated builds

Signed-off-by: Simon McVittie <smcv@collabora.com>

.gitignore

@@ -3,6 +3,7 @@
 *.rej
 *.o
 *~
+/.ccache/
 /build-aux/
 compile
 config.cache

.gitlab-ci.yml

@@ -27,6 +27,16 @@ stages:
 
 before_script:
   - ./tools/ci-install.sh
+  - mkdir -p .ccache
+  - export CCACHE_BASEDIR="$(pwd)"
+  - export CCACHE_DIR="$CCACHE_BASEDIR/.ccache"
+  # Debian's ccache package creates symlinks here for all supported
+  # compilers
+  - export PATH="/usr/lib/ccache:$PATH"
+
+cache:
+  paths:
+    - .ccache/
 
 variables:
   ci_in_docker: "yes"

tools/ci-install.sh

@@ -126,6 +126,7 @@ case "$ci_distro" in
             autoconf-archive \
             automake \
             autotools-dev \
+            ccache \
             cmake \
             debhelper \
             dh-autoreconf \