ci: Set -Werror for all CI builds

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

Author: real-or-random

.cirrus.yml

@@ -1,21 +1,25 @@
 env:
-  WIDEMUL: auto
+  ## compiler options
+  HOST:
+  WERROR_CFLAGS: -Werror # Specific warnings can be disabled with -Wno-error=foo.
+  MAKEFLAGS: -j2
+  EXTRAFLAGS:
+  BUILD: check
+  # secp256k1 config
   STATICPRECOMPUTATION: yes
   ECMULTGENPRECISION: auto
   ASM: no
-  BUILD: check
+  WIDEMUL: auto
   WITH_VALGRIND: yes
   RUN_VALGRIND: no
-  EXTRAFLAGS:
-  HOST:
-  ECDH: no
-  RECOVERY: no
-  SCHNORRSIG: no
-  EXPERIMENTAL: no
   CTIMETEST: yes
   BENCH: yes
   ITERS: 2
-  MAKEFLAGS: -j2
+  # secp256k1 modules
+  EXPERIMENTAL: no
+  ECDH: no
+  RECOVERY: no
+  SCHNORRSIG: no
 
 cat_logs_snippet: &CAT_LOGS
   always:
@@ -207,6 +211,8 @@ task:
     WINE_CMD: wine64-stable
     HOST: x86_64-w64-mingw32
     BUILD:
+    # https://github.com/bitcoin-core/secp256k1/issues/923
+    WERROR_CFLAGS: -Werror -Wno-error=attributes
     WITH_VALGRIND: no
     ECDH: yes
     RECOVERY: yes