Another VPATH patch for ecpg

Hi,

I just detected another problem with building ecpg in a VPATH
environment.  This patch fixes it for me.

This is needed because ecpg_config.h ends up in the build dir rather
than the source dir.

Thanks,

--
Alvaro Herrera                                http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.