Add separate target to the build system for checksum backend linkage. This
eliminates the need of adding openssl directly (which acted in the same way
before), only it's more convinient for other backends like Windows BCrypt.

* CMakeLists.txt
  (checksum backend): Setup backends via external-checksum-libs target.
* build/generator/gen_win_dependencies.py
  (GenDependenciesBase): Ignore 'checksum-libs'.
* build.conf
  (libsvn_subr): Link against checksum-libs
  (checksum-libs): New target.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1931390 13f79535-47bb-0310-9956-ffa450edef68