configure: add '-psql_config' option

configure: add '-psql_config' option

Allow setting of pg_config path for cross compilation where pg_config is
not in the command search path (do the same as for mysql_config).
This is e.g. used for buildroot (see [1] for details).

Patch is backported from qtbase/a59028d6e98bcf5a13c9103753e5e83c362c63bc.

[1] http://lists.busybox.net/pipermail/buildroot/2015-March/121654.html

Change-Id: If2cce9c8e9e3f87bcbe80fcccacad881ec14b476
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>