Allow using :scriptdir variable in regress tests

Have legacy.sql use it to find legacy.sql.
The run_test.pl script sets it using pg_config when --extensions
switch is given.

git-svn-id: http://svn.osgeo.org/postgis/trunk@12375 b70326c6-7e19-0410-871a-916f4a2858ee
Have regress/legacy.sql truncate spatial_ref_sys

git-svn-id: http://svn.osgeo.org/postgis/trunk@12376 b70326c6-7e19-0410-871a-916f4a2858ee