Enable Python Py_DEBUG setting when built with --enable-dbgutil on Linux

This has been suggested on IRC for testing fixes/avoiding regressions
when working on the GIL locking in PyUNO.

Change-Id: Ifda21a867b3c0c7db636a9ec950050012e4742de
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/106791
Reviewed-by: Michael Stahl <michael.stahl@cib.de>
Tested-by: Jenkins

external/python3/ExternalProject_python3.mk

@@ -87,6 +87,7 @@ $(call gb_ExternalProject_get_state_target,python3,build) :
 		./configure \
 		$(if $(CROSS_COMPILING),--build=$(BUILD_PLATFORM) --host=$(HOST_PLATFORM)) \
 		$(if $(ENABLE_VALGRIND),--with-valgrind) \
+		$(if $(ENABLE_DBGUTIL),--with-pydebug) \
 		--prefix=/python-inst \
 		--with-system-expat \
 		$(if $(filter AIX,$(OS)), \