Errors if gperf isn't installed

[David Zeuthen]

On Wed, 2007-06-06 at 11:47 +0100, Bastien Nocera wrote:
> I had some trouble yesterday with gperf not being installed but creating
> a .h files anyway.
> 
> This patch makes gperf itself create the file, so it would error out
> properly if it wasn't there.

This patch only gets us halfway there; for example it still breaks if
you run 'make clean'. Basically 'make distcheck' should pass even if
gperf is not installed.

     David