sal: rtlbootstrap.mk was empty on cygwin

This needs proper fix. It wasn't noticed because it was created by pipe
before afd5eb1f and last command
succeeded, so make continued even if compilation failed.

sal/CustomTarget_generated.mk

@@ -33,7 +33,12 @@ $(call gb_CustomTarget_get_target,sal/generated) : \
 	$(SAUT)/rtlbootstrap.mk $(SAUT)/sal/udkversion.h \
 	$(if $(filter-out $(COM),MSC),$(SAUT)/sal/typesizes.h)
 
-$(SAUT)/rtlbootstrap.mk : $(if $(filter-out $(COM),MSC),$(SAUT)/sal/typesizes.h) | $(SAUT)/.dir
+# FIXME: rtlbootstrap.mk is empty on cygwin
+ifeq ($(COM),MSC)
+$(SAUT)/rtlbootstrap.mk :| $(SAUT)/.dir
+	touch $@
+else
+$(SAUT)/rtlbootstrap.mk : $(SAUT)/sal/typesizes.h | $(SAUT)/.dir
 	$(call gb_Output_announce,$(subst $(WORKDIR)/,,$@),$(true),CXX,1)
 	$(call gb_Helper_abbreviate_dirs_native, \
 	(echo '#include "macro.hxx"'; echo RTL_OS:=THIS_OS; echo RTL_ARCH:=THIS_ARCH) > $(SAUT)/bootstrap.cxx && \
@@ -41,6 +46,7 @@ $(SAUT)/rtlbootstrap.mk : $(if $(filter-out $(COM),MSC),$(SAUT)/sal/typesizes.h)
 		-I$(SRCDIR)/sal/inc $(SAUT)/bootstrap.cxx > $@.tmp && \
 	cat $@.tmp | grep '^RTL_' | sed -e 's/"//g' > $@ && \
 	rm $(SAUT)/bootstrap.cxx $@.tmp)
+endif
 
 $(SAUT)/sal/typesizes.h :| $(SAUT)/sal/.dir
 	$(call gb_Output_announce,$(subst $(WORKDIR)/,,$@),$(true),ECH,1)