ci: add -Werror to CFLAGS, added clang build target

.gitlab-ci.yml

@@ -15,8 +15,8 @@ stages:
 build:linux:amd64:
   stage: build
   script:
-    - PREFIX=$(pwd)/.local make -k all
-    - PREFIX=$(pwd)/.local make install
+    - PREFIX=$(pwd)/.local make -k all CFLAGS=-Werror
+    - PREFIX=$(pwd)/.local make install CFLAGS=-Werror
   artifacts:
     untracked: true
   tags:
@@ -24,6 +24,16 @@ build:linux:amd64:
     - linux
     - amd64
 
+build:clang:linux:amd64:
+  stage: build
+  script:
+    - CXX=clang++ CC=clang PREFIX=$(pwd)/.local make -k all CFLAGS=-Werror
+    - CXX=clang++ CC=clang PREFIX=$(pwd)/.local make install CFLAGS=-Werror
+  tags:
+    - docker
+    - linux
+    - amd64
+
 lint:lua:
   stage: test
   dependencies: []  # do not download build artifacts

ci/Dockerfile

@@ -69,3 +69,6 @@ RUN luarocks install luacov
 
 # LuaJIT binary for stand-alone scripting
 RUN apt-get install -y -qqq luajit
+
+# clang for kresd CI
+RUN apt-get install -y -qqq clang