cmake: Clarify error message for invalid SVN_CHECKSUM_BACKEND value.

* CMakeLists.txt: 
  (): Mention 'bcrypt' and 'xdigest' as possible values for
      SVN_CHECKSUM_BACKEND property.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1936000 13f79535-47bb-0310-9956-ffa450edef68

Author: Ivan Zhakov

CMakeLists.txt

@@ -653,7 +653,7 @@ elseif (SVN_CHECKSUM_BACKEND STREQUAL "xdigest")
 else()
   message(SEND_ERROR
     "Invalid value of SVN_CHECKSUM_BACKEND: '${SVN_CHECKSUM_BACKEND}'. "
-    "Possible options are 'apr' or 'openssl'."
+    "Possible options are 'apr', 'openssl', 'bcrypt', 'xdifest'."
   )
 endif()