Fix swig bindings with out of tree builds when in release mode.

When ./autogen.sh is run with --release then we run SWIG to generate
the C files then rather than at build time.  We do this when building
tarballs.  Currently those source files are flagged in our build as
generated so they are expected to be under the build directory.  When
in source mode flag them as not generated so we look for them in the
source directory.

* build/gen_base.py
  (SWIGObject.__init__): Add a flag to specify if we're in release mode and to
    set the source_generated flag appropriately.
  (TargetSWIG.add_dependencies): Pass the release mode flag when creating a
    SWIGObject.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1543187 13f79535-47bb-0310-9956-ffa450edef68