Fix the output of an informational message that we output when
changing the 'shebang line' in installable scripts. [Robin Barker]Backported a change from the 0.27_xx branch that keeps track of any
custom Module::Build subclass that might have been created in the
Build.PL script, so that we can reinstantiate the right class in
new_from_context(). [Requested by Jos Boumans]