glew depends on OpenCL, for tests too

Change-Id: Ib591b4328589982cda762ce2b1a688345b1665b0
Reviewed-on: https://gerrit.libreoffice.org/13652Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

sc/CppunitTest_sc_opencl_test.mk

@@ -22,7 +22,7 @@ $(eval $(call gb_CppunitTest_use_externals,sc_opencl_test, \
 
 $(eval $(call gb_CppunitTest_use_libraries,sc_opencl_test, \
     basegfx \
-	clew \
+    $(call gb_Helper_optional,OPENCL,clew) \
     comphelper \
     cppu \
     cppuhelper \