Enable ccache for the linux*make*clang job.

Author: NlightNFotis

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

@@ -62,8 +62,8 @@ jobs:
   check-ubuntu-20_04-make-clang:
     runs-on: ubuntu-20.04
     env:
-      CC: "/usr/bin/clang"
-      CXX: "/usr/bin/clang++"
+      CC: "ccache /usr/bin/clang"
+      CXX: "ccache /usr/bin/clang++"
     steps:
       - uses: actions/checkout@v2
         with:
@@ -75,20 +75,36 @@ jobs:
           DEBIAN_FRONTEND: noninteractive
         run: |
           sudo apt-get update
-          sudo apt-get install --no-install-recommends -yq clang-10 clang++-10 gdb maven jq flex bison libxml2-utils cpanminus
+          sudo apt-get install --no-install-recommends -yq clang-10 clang++-10 gdb maven jq flex bison libxml2-utils cpanminus ccache
           make -C src minisat2-download
           cpanm Thread::Pool::Simple
+      - name: Prepare ccache
+        uses: actions/cache@v2
+        with:
+          path: .ccache
+          key: ${{ runner.os }}-20.04-make-clang-${{ github.ref }}-${{ github.sha }}-PR
+          restore-keys: |
+            ${{ runner.os }}-20.04-make-clang-${{ github.ref }}
+            ${{ runner.os }}-20.04-make-clang
+      - name: ccache environment
+        run: |
+          echo "CCACHE_BASEDIR=$PWD" >> $GITHUB_ENV
+          echo "CCACHE_DIR=$PWD/.ccache" >> $GITHUB_ENV
+      - name: Zero ccache stats and limit in size
+        run: ccache -z --max-size=500M
       - name: Build with make
         run: |
           make -C src       -j2
           make -C unit      -j2
           make -C jbmc/src  -j2
           make -C jbmc/unit -j2
+      - name: Print ccache stats
+        run: ccache -s
       - name: Run unit tests
         run: |
           make -C unit      test
           make -C jbmc/unit test
-      - name: Running expected failure unit tests
+      - name: Run expected failure unit tests
         run: |
           make TAGS="[!shouldfail]" -C unit test
           make TAGS="[!shouldfail]" -C jbmc/unit test
@@ -97,7 +113,8 @@ jobs:
           make -C regression test-parallel JOBS=2
           make -C regression/cbmc test-paths-lifo
           env PATH=$PATH:`pwd`/src/solvers make -C regression/cbmc test-cprover-smt2
-          make -C jbmc/regression test-parallel JOBS=2 
+          make -C jbmc/regression test-parallel JOBS=2
+
   check-ubuntu-20_04-cmake-gcc:
     runs-on: ubuntu-20.04
     steps: