Add a new GLWRAP_DEFER_WITH_RETURN macro.

Add a new GLWRAP_DEFER_WITH_RETURN macro.

This allows for the elimination of some code duplication from our
implementation of glXGetPrcAddressARB. The previous implementation
duplicated code from glwrap_lookup simply because the GLWRAP_DEFER
macro did not provide access to the return value of the wrapped
function.

With the new macro, (very much like GLWRAP_DEFER but accepting a
parameter for a variable to accept the return value), we can eliminate
this code duplication.

Of course, our symbol-extraction script is now a bit more complicated
since it has to find occurrences of DEFER_WITH_RETURN in addition to
occurrences of DEFER, and pull out the function name as the second
argument rather than the first.