ci: Build with --enable-werror by default, and document exceptions

ci/test/00_setup_env_arm.sh

@@ -25,4 +25,4 @@ export RUN_FUNCTIONAL_TESTS=false
 export GOAL="install"
 # -Wno-psabi is to disable ABI warnings: "note: parameter passing for argument of type ... changed in GCC 7.1"
 # This could be removed once the ABI change warning does not show up by default
-export BITCOIN_CONFIG="--enable-glibc-back-compat --enable-reduce-exports CXXFLAGS=-Wno-psabi --enable-werror --with-boost-process"
+export BITCOIN_CONFIG="--enable-glibc-back-compat --enable-reduce-exports CXXFLAGS=-Wno-psabi --with-boost-process"

ci/test/00_setup_env_mac.sh

@@ -15,4 +15,4 @@ export XCODE_BUILD_ID=11C505
 export RUN_UNIT_TESTS=false
 export RUN_FUNCTIONAL_TESTS=false
 export GOAL="deploy"
-export BITCOIN_CONFIG="--with-gui --enable-reduce-exports --enable-werror --with-boost-process"
+export BITCOIN_CONFIG="--with-gui --enable-reduce-exports --with-boost-process"

ci/test/00_setup_env_mac_host.sh

@@ -9,7 +9,7 @@ export LC_ALL=C.UTF-8
 export HOST=x86_64-apple-darwin18
 export PIP_PACKAGES="zmq"
 export GOAL="install"
-export BITCOIN_CONFIG="--with-gui --enable-reduce-exports --enable-werror --with-boost-process"
+export BITCOIN_CONFIG="--with-gui --enable-reduce-exports --with-boost-process"
 export CI_OS_NAME="macos"
 export NO_DEPENDS=1
 export OSX_SDK=""

ci/test/00_setup_env_win64.sh

@@ -14,3 +14,7 @@ export RUN_FUNCTIONAL_TESTS=false
 export RUN_SECURITY_TESTS="true"
 export GOAL="deploy"
 export BITCOIN_CONFIG="--enable-reduce-exports --disable-gui-tests --without-boost-process"
+
+# Compiler for MinGW-w64 causes false -Wreturn-type warning.
+# See https://sourceforge.net/p/mingw-w64/bugs/306/
+export NO_WERROR=1

ci/test/06_script_a.sh

@@ -6,7 +6,10 @@
 
 export LC_ALL=C.UTF-8
 
-BITCOIN_CONFIG_ALL="--disable-dependency-tracking --prefix=$DEPENDS_DIR/$HOST --bindir=$BASE_OUTDIR/bin --libdir=$BASE_OUTDIR/lib"
+BITCOIN_CONFIG_ALL="--enable-suppress-external-warnings --disable-dependency-tracking --prefix=$DEPENDS_DIR/$HOST --bindir=$BASE_OUTDIR/bin --libdir=$BASE_OUTDIR/lib"
+if [ -z "$NO_WERROR" ]; then
+  BITCOIN_CONFIG_ALL="${BITCOIN_CONFIG_ALL} --enable-werror"
+fi
 DOCKER_EXEC "ccache --zero-stats --max-size=$CCACHE_SIZE"
 
 BEGIN_FOLD autogen