...similarly to how it is used for other external libraries since
f0aa1a78 "For Clang -fsanitize=vptr use
-fvisibility-ms-compat, not -fvisibility=hidden".  This caused
CppunitTest_writerperfect_writer to fail now with -fsanitize=vptr after
7915f35d "EPUB export: implement doc/para
open/close and text in doc handler", where additions to
writerperfect/source/writer/EPUBExportFilter.cxx caused Library_wpftwriter to
import more stuff from the libepubgen archive, which happend to make it pick up
the (erroneously hidden) definition of _ZTIN10librevenge17RVNGTextInterfaceE
from that archive, instead of from some other object/archive as happened before.

Change-Id: I2b56df38d7225505ff993321598ddd51812d9de4