Add costs to functions in the form of macros:

Add costs to functions in the form of macros:
_COST_LOW, _COST_MEDIUM, _COST_HIGH
Macros are set in sqldefines.h.in and are switched
on PgSQL version. Prior to PG12, costs should be
small or nonexistent, to avoid SQL inlining issues.
After Pg12, costs can be higher to hopefully
force more parallelism in plans.
Closes #4344

git-svn-id: http://svn.osgeo.org/postgis/trunk@17307 b70326c6-7e19-0410-871a-916f4a2858ee