Fix undefined symbols for plugins.

Index: src/CMakeLists.txt
--- src/CMakeLists.txt.orig
+++ src/CMakeLists.txt
@@ -280,6 +280,8 @@ if (UNIX AND NOT APPLE)
 endif ()
 
 
+SET_TARGET_PROPERTIES(${EXE_NAME} PROPERTIES LINK_FLAGS -rdynamic)
+
 # For OS X, generate `Info.plist` and add it to the `.app` bundle.
 IF (APPLE)
   SET(VER ${TeXworks_VERSION})
