Set PYTHONWARNINGS to error by default for --enable-werror

Setting it in environment overrides this setting.
The rationale is to avoid introducing warnings like these appeared recently:

    zipfile.py:1517: UserWarning: Duplicate name: 'cmd/ar/sc_bulletsandnumberingdialog.png'

(see e.g. https://ci.libreoffice.org/job/gerrit_windows/71910/consoleFull)

Change-Id: I8ae42e039ec3d028c01dbc4bcf422feae9e46271
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/100268
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>

config_host.mk.in

@@ -499,6 +499,7 @@ export PYTHON_LIBS=$(gb_SPACE)@PYTHON_LIBS@
 export PYTHON_VERSION=@PYTHON_VERSION@
 export PYTHON_VERSION_MAJOR=@PYTHON_VERSION_MAJOR@
 export PYTHON_VERSION_MINOR=@PYTHON_VERSION_MINOR@
+export PYTHONWARNINGS=@PYTHONWARNINGS@
 export QRCODEGEN_CFLAGS=$(gb_SPACE)@QRCODEGEN_CFLAGS@
 export QRCODEGEN_LIBS=$(gb_SPACE)@QRCODEGEN_LIBS@
 export QT5_CFLAGS=$(gb_SPACE)@QT5_CFLAGS@

configure.ac

@@ -5136,16 +5136,19 @@ dnl ===================================================================
 AC_MSG_CHECKING([whether to turn warnings to errors])
 if test -n "$enable_werror" -a "$enable_werror" != "no"; then
     ENABLE_WERROR="TRUE"
+    PYTHONWARNINGS="error"
     AC_MSG_RESULT([yes])
 else
     if test -n "$LODE_HOME" -a -z "$enable_werror"; then
         ENABLE_WERROR="TRUE"
+        PYTHONWARNINGS="error"
         AC_MSG_RESULT([yes])
     else
         AC_MSG_RESULT([no])
     fi
 fi
 AC_SUBST(ENABLE_WERROR)
+AC_SUBST(PYTHONWARNINGS)
 
 dnl Check for --enable-assert-always-abort, set ASSERT_ALWAYS_ABORT
 dnl ===================================================================

external/icu/ExternalProject_icu.mk

@@ -21,7 +21,7 @@ ifeq ($(OS),WNT)
 $(call gb_ExternalProject_get_state_target,icu,build) :
 	$(call gb_Trace_StartRange,icu,EXTERNAL)
 	$(call gb_ExternalProject_run,build,\
-		export LIB="$(ILIB)" \
+		export LIB="$(ILIB)" PYTHONWARNINGS="default" \
 		&& CFLAGS="-FS $(SOLARINC) $(gb_DEBUGINFO_FLAGS)" CPPFLAGS="$(SOLARINC)" CXXFLAGS="-FS $(SOLARINC) $(gb_DEBUGINFO_FLAGS)" \
 			INSTALL=`cygpath -m /usr/bin/install` \
 			./runConfigureICU \
@@ -65,6 +65,7 @@ $(call gb_ExternalProject_get_state_target,icu,build) :
 	$(call gb_ExternalProject_run,build,\
 		CPPFLAGS=$(icu_CPPFLAGS) CFLAGS=$(icu_CFLAGS) \
 		CXXFLAGS=$(icu_CXXFLAGS) LDFLAGS=$(icu_LDFLAGS) \
+		PYTHONWARNINGS="default" \
 		./configure \
 			--disable-layout --disable-samples \
 			$(if $(filter FUZZERS,$(BUILD_TYPE)),--disable-release) \