Fix deadlock when indirectly calling PLCObject's evaluator() from twisted of wx event loops. Was freezing while transfer/start/stop through Wamp. Serialize all PLCObject's call to self.PythonRuntimeCall into the same PythonThread thread.
<xsd:attribute name="Compiler" type="xsd:string" use="optional" default="gcc"/>
<xsd:attribute name="CFLAGS" type="xsd:string" use="optional" default=""/>
<xsd:attribute name="Linker" type="xsd:string" use="optional" default="gcc"/>
<xsd:attribute name="LDFLAGS" type="xsd:string" use="optional" default=""/>