Fix usage of WITH_SUBVERSION_VER
devel/subversion used to be available in different versions from which the
end-user could choose by setting WITH_SUBVERSION_VER to the expected version.
This has been reworked at some point and broke that feature in portshaker.
Adjust the WITH_SUBVERSION_VER handling to cope with it again.
PR: 250849
Submitted by: nork@ninth-nine.com