libmerged: fix linking against cppunit again

Fix for: ./program/soffice.bin: error while loading shared libraries:
libcppunit-1.13.so.0: cannot open shared object file: No such file or
directory

Change-Id: Ib73010f69c7fcab0e5759df9e77e0848aa80579f

Library_merged.mk

@@ -46,7 +46,6 @@ $(eval $(call gb_Library_use_libraries,merged,\
 $(eval $(call gb_Library_use_externals,merged,\
 	boostdatetime \
 	$(call gb_Helper_optional,DESKTOP,clucene) \
-	cppunit \
 	cups \
 	curl \
 	expat \
@@ -58,14 +57,20 @@ $(eval $(call gb_Library_use_externals,merged,\
 	lcms2 \
 	libxml2 \
 	libxslt \
-	libexslt \
 	$(if $(filter-out IOS,$(OS)),lpsolve) \
 	mythes \
 	nss3 \
-	$(call gb_Helper_optional,PYUNO,python) \
 	zlib \
 ))
 
+ifeq (ALL,$(MERGELIBS))
+$(eval $(call gb_Library_use_externals,merged,\
+	cppunit \
+	libexslt \
+	$(call gb_Helper_optional,PYUNO,python) \
+))
+endif
+
 ifeq ($(ENABLE_GRAPHITE),TRUE)
 $(eval $(call gb_Library_use_externals,merged,\
 	graphite \