Provide some initial support for building the ODBC driver for

Provide some initial support for building the ODBC driver for
an already installed iODBC or unixODBC driver manager.  In particular,
use the include files provided by the driver manager over our own,
and use the odbcinst library of the driver manager rather than gpps.c.

Migrate portability sections common to several files into psqlodbc.h.