Add -Wl,--export-dynamic linker flag

This seems required for GtkBuilder to be able to connect signals. However, this change may break Windows
builds. Needs to be tested and worked around if it is the case.

wscript

@@ -325,6 +325,7 @@ def build(bld):
         source          = geany_sources,
         includes        = ['.', 'scintilla/include/', 'tagmanager/include/'],
         defines         = ['G_LOG_DOMAIN="Geany"', 'GEANY_PRIVATE'],
+        linkflags       = ['-Wl,--export-dynamic'],
         uselib          = ['GTK', 'GLIB', 'GIO', 'GTHREAD', 'WIN32', 'SUNOS_SOCKET'],
         use             = ['scintilla', 'tagmanager', 'mio'])