DIST_SUBDIRS should not be inside PYTHON_ENABLED conditional

Build (at least on centos8) fails when DIST_SUBDIRS is inside conditional. Moving it out solves the problem.

According to @waananen: Whether or not you build submodules should not have an impact of the dist tarball you build. Therefore DIST_SUBDIRS SHOULD not be inside conditional while SUBDIRS should where appropriate. One is for "what is DISTributed" and the other is for building.