Changes needed to successfully build opensp with the GNU compilers.

Taken from the fix at:

This change is already known upstream.

--- OpenSP-1.5.1/include/InternalInputSource.h.orig 2016-09-19 12:19:35.766064218 +0000

+++ OpenSP-1.5.1/include/InternalInputSource.h 2016-09-19 12:20:04.849794945 +0000

@@ -34,7 +34,7 @@

void pushCharRef(Char ch, const NamedCharRef &);

Boolean rewind(Messenger &);

const StringC *contents();

- InternalInputSource *InternalInputSource::asInternalInputSource();

+ InternalInputSource *asInternalInputSource();

~InternalInputSource();

private:

InternalInputSource(const InternalInputSource &); // undefined