Makefile: add entries for windows build

Makefile.builder

@@ -1,3 +1,5 @@
 ifeq ($(PACKAGE_SET),dom0)
 RPM_SPEC_FILES := $(addprefix rpm_spec/,core-dom0.spec core-dom0-doc.spec)
+WIN_SOURCE_SUBDIRS := .
+WIN_COMPILER := mingw
 endif