ci: Make compiler warning into errors on CI

This also tidies the list of environment variables in .cirrus.yml.

.cirrus.yml

@@ -1,21 +1,28 @@
 env:
-  WIDEMUL: auto
+  ### compiler options
+  HOST:
+  # Specific warnings can be disabled with -Wno-error=foo.
+  # -pedantic-errors is not equivalent to -Werror=pedantic and thus not implied by -Werror according to the GCC manual.
+  WERROR_CFLAGS: -Werror -pedantic-errors
+  MAKEFLAGS: -j2
+  BUILD: check
+  ### secp256k1 config
   STATICPRECOMPUTATION: yes
   ECMULTGENPRECISION: auto
   ASM: no
-  BUILD: check
+  WIDEMUL: auto
   WITH_VALGRIND: yes
   EXTRAFLAGS:
-  HOST:
+  ### secp256k1 modules
+  EXPERIMENTAL: no
   ECDH: no
   RECOVERY: no
   SCHNORRSIG: no
-  EXPERIMENTAL: no
-  CTIMETEST: yes
-  BENCH: yes
+  ### test options
   TEST_ITERS:
+  BENCH: yes
   BENCH_ITERS: 2
-  MAKEFLAGS: -j2
+  CTIMETEST: yes
 
 cat_logs_snippet: &CAT_LOGS
   always:

configure.ac

@@ -357,6 +357,9 @@ if test x"$enable_valgrind" = x"yes"; then
   SECP_INCLUDES="$SECP_INCLUDES $VALGRIND_CPPFLAGS"
 fi
 
+# Add -Werror and similar flags passed from the outside (for testing, e.g., in CI)
+SECP_CFLAGS="$SECP_CFLAGS $WERROR_CFLAGS"
+
 # Handle static precomputation (after everything which modifies CFLAGS and friends)
 if test x"$use_ecmult_static_precomputation" != x"no"; then
   if test x"$cross_compiling" = x"no"; then