Patch to fix parallel build of extensions

[Bas Couwenberg]

Building postgis with -j2 or more fails due to extensions not being parallel safe.

The directories are not created for all scripts in those directories.

The postgis Debian package was recently updated to enable parallel builds. The initial build failed due to the postgis_tiger_geocoder extension and the second build failed due to the postgis exentions, the cause was always missing mkdir commands in the target in question.

The attached patch fixes the issue.

[pramsey]

In 16665:

Better parallel build, closes #4105

[pramsey]

In 16667:

Better parallel build (references #4105)