Configure build+test matrix for GitLab CI:

* Disable parallel tests for address sanitizer
* Improve caching
* Prefer Ninja builds because they are faster

bin/ci/build.sh

@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+
+set -o xtrace
+set -o errexit
+
+# The build system. Either 'Unix Makefiles' or 'Ninja'.
+GENERATOR=${GENERATOR:-Unix Makefiles}
+# The compiler. Either 'gcc' or 'clang'.
+COMPILER=${COMPILER:-gcc}
+# The build type. Either 'Debug' or 'Release'.
+BUILD_TYPE=${BUILD_TYPE:-Debug}
+# Additional arguments to CMake.
+# We use the `-` substitution here instead of `:-` so that callers can erase
+# the default by setting `$CMAKE_ARGS` to the empty string.
+CMAKE_ARGS=${CMAKE_ARGS-'-Dwerr=ON'}
+
+if [[ ${COMPILER} == 'gcc' ]]; then
+  export CC='gcc'
+  export CXX='g++'
+elif [[ ${COMPILER} == 'clang' ]]; then
+  export CC='clang'
+  export CXX='clang++'
+fi
+
+mkdir build
+cd build
+cmake -G "${GENERATOR}" -DCMAKE_BUILD_TYPE=${BUILD_TYPE} ${CMAKE_ARGS} ..
+cmake --build . -- -j $(nproc)