Make warnings from C fatal.

.gitlab-ci.yml

@@ -135,7 +135,7 @@ run_test_gcc_9_1:
       ./configure
       CC=mpicc FC=mpifort
       FCFLAGS="-O2 -march=native -cpp -Wall -Wextra"
-      CFLAGS="-O2 -march=native -Wall -Wextra"
+      CFLAGS="-O2 -march=native -Wall -Wextra -Werror"
       --disable-silent-rules
 # build yaxt and unit tests without running tests
     - make -j8 2> >(tee make.err)