allow system-libepubgen to limp along

Change-Id: I038421749ed6f20d74984beb2e703ffa43a09ff3
Reviewed-on: https://gerrit.libreoffice.org/42620Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

writerperfect/source/writer/EPUBExportFilter.cxx

@@ -25,6 +25,17 @@
 
 using namespace com::sun::star;
 
+#if !LIBEPUBGEN_VERSION_SUPPORT
+namespace libepubgen
+{
+    enum EPUBStylesMethod
+    {
+        EPUB_STYLES_METHOD_CSS, //< The styles will be described in a seprarate CSS file.
+        EPUB_STYLES_METHOD_INLINE, //< The styles will be described inline.
+    };
+}
+#endif
+
 namespace writerperfect
 {