Further SYSTEM_MOZILLA/SYSTEM_NSS clean-up

...past 8648c974 "Clean-up 'SYSTEM_MOZILLA' ->
'SYSTEM_NSS.'"  Looks like -DSYSTEM_MOZILLA in RepositoryExternal.mk's
gb_LinkTarget__use_nss3 was only relevant for
xmlsecurity/source/xmlsec/nss/nssinitializer.cxx (for which it had been set
explicitly in xmlsecurity/Library_xsec_xmlsec.mk, and renamed to SYSTEM_NSS
already).  And looks like xmlsecurity/Library_xsec_xmlsec.mk's explicit
SYSTEM_NSS code was more or less a duplication of RepositoryExternal.mk's
gb_LinkTarget__use_nss3, so made that DRY.

Change-Id: Ie6eca2c4eb7fb70f6a36c97d8681b4140f9870cb

RepositoryExternal.mk

@@ -2153,7 +2153,7 @@ ifeq ($(SYSTEM_NSS),YES)
 
 define gb_LinkTarget__use_nss3
 $(call gb_LinkTarget_add_defs,$(1),\
-	-DSYSTEM_MOZILLA \
+	-DSYSTEM_NSS \
 )
 
 $(call gb_LinkTarget_set_include,$(1),\

xmlsecurity/Library_xsec_xmlsec.mk

@@ -70,6 +70,7 @@ $(eval $(call gb_Library_use_libraries,xsec_xmlsec,\
 
 $(eval $(call gb_Library_use_externals,xsec_xmlsec,\
 	libxml2 \
+	nss3 \
 ))
 
 $(eval $(call gb_Library_add_exception_objects,xsec_xmlsec,\
@@ -148,36 +149,6 @@ $(eval $(call gb_Library_add_exception_objects,xsec_xmlsec,\
 
 endif # ifeq ($(GUI)$(CROSS_COMPILING),WNT)
 
-ifeq ($(SYSTEM_NSS),YES)
-
-$(eval $(call gb_Library_set_include,xsec_xmlsec,\
-	$$(INCLUDE) \
-	$(NSS_CFLAGS) \
-))
-
-$(eval $(call gb_Library_add_defs,xsec_xmlsec,\
-	-DSYSTEM_NSS \
-))
-
-$(eval $(call gb_Library_add_libs,xsec_xmlsec,\
-	$(NSS_LIBS) \
-))
-
-else
-
-$(eval $(call gb_Library_set_include,xsec_xmlsec,\
-	$$(INCLUDE) \
-	-I$(OUTDIR)/inc/mozilla/nspr \
-	-I$(OUTDIR)/inc/mozilla/nss \
-))
-
-$(eval $(call gb_Library_use_libraries,xsec_xmlsec,\
-	nspr4 \
-	nss3 \
-))
-
-endif # ifeq ($(SYSTEM_NSS),YES)
-
 ifeq ($(OS),SOLARIS)
 $(eval $(call gb_Library_add_libs,xsec_xmlsec,\
 	-ldl \