make clean does not drop the sql/ dirs

[strk]

If you happen to build postgis in source tree and then want to build elsewhere, running make distclean will set you in trouble as it will leave a few empty sql/ directory in the source tree so then when building in a build tree the VPATH will think 'sql' target exists and not create it again, resulting in errors like

root@5fc637b9a827:/postgis/postgis/build/pg12/extensions/postgis# make sql/postgis_for_extension.sql
/usr/bin/cpp -traditional-cpp -w -P -Upixel -Ubool -I./../../postgis ../../../../extensions/postgis/../../postgis/postgis.sql.in > sql/postgis_for_extension.sql.tmp
/bin/sh: 1: cannot create sql/postgis_for_extension.sql.tmp: Directory nonexistent

We'd better remove those empty directories too on make clean

[strk]

I cannot reproduce: the only sql/ dirs left after make distclean for me are those under topology/ and libpgcommon/ and are both non-empty and contain source files.

This is in stable-3.4 and master branches

I'll close and in case it gets back I'll reopen or file a new one