ci: test headerscheck, cpluspluscheck as part of CompilerWarnings task.

Discussion: https://postgr.es/m/20220323002024.f2g6tivduzrktgfa@alap3.anarazel.de

.cirrus.yml

@@ -576,5 +576,28 @@ task:
       make -s -j${BUILD_JOBS} clean
       time make -s -j${BUILD_JOBS} -C doc
 
+  ###
+  # Verify headerscheck / cpluspluscheck succeed
+  #
+  # - Don't use ccache, the files are uncacheable, polluting ccache's
+  #   cache
+  # - Use -fmax-errors, as particularly cpluspluscheck can be very verbose
+  # - XXX have to disable ICU to avoid errors:
+  #   https://postgr.es/m/20220323002024.f2g6tivduzrktgfa%40alap3.anarazel.de
+  # - XXX: the -Wno-register avoids verbose warnings:
+  #   https://postgr.es/m/20220308181837.aun3tdtdvao4vb7o%40alap3.anarazel.de
+  ###
+  always:
+    headers_headerscheck_script: |
+      time ./configure \
+        ${LINUX_CONFIGURE_FEATURES} \
+        --without-icu \
+        --quiet \
+        CC="gcc" CXX"=g++" CLANG="clang"
+      make -s -j${BUILD_JOBS} clean
+      time make -s headerscheck EXTRAFLAGS='-fmax-errors=10'
+    headers_cpluspluscheck_script: |
+      time make -s cpluspluscheck EXTRAFLAGS='-Wno-register -fmax-errors=10'
+
   always:
     upload_caches: ccache