Improve/fix libtool library versioning

[notroj]

* configure.ac: Pass complete library version information to libtool
  via $SVN_LT_SOVERSION, so library filenames are unique to each
  release rather than constant, while keeping the sonames intact.

  Examples for SVN 1.14.5:
   Filename before:          libsvn_subr-1.so.0.0.0
   Filename after:           libsvn_subr-1.so.0.14.5
   SONAME before and after:  libsvn_subr-1.so.0