version dependent compilation

Apparently, there's currently no way to perform conditional compiling 
dependent on the version of pgsql. Currently we're facing the problem 
that ParseDateTime changed its parameters between 8.0.3 and 8.0.4, 
breaking backward compatibility (for good reasons in this case).

IMHO it's quite helpful to have macros like

#define PGSQL_VERSION_MAJOR    8
#define PGSQL_VERSION_MINOR    0
#define PGSQL_VERSION_POINT    4

or whatever naming scheme seems convenient that allow to catch such 
deviations without having to maintain version specific sources manually.
Could be added for  8.1.0, 8.0.5, 7.4.10.

Regards,
Andreas