Simplify passing of configure arguments to pg_config

Currently, configure puts the configure args into the makefiles and
then have the makefiles pass them to the build of pg_config.  That looks 
like an unnecessary redirection, and indeed that method was
put in place when pg_config was a shell script.  We can simplify that
by having configure put the value into pg_config.h directly.  This
also makes the standard build system match how the MSVC build system
already does it.

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services