Revision 1878379

configure: Add new option to specify path to swig bindings targets

Before this commit, it was not possible to use distinct Python
installations for the build system and test suite on the one hand,
and for building language bindings against on the other hand.

So we introduce new variable "SWIG_PY_PYTHON" for a target of
the SWIG Python bindings and a configure option to specify it. 
Also, for the symmetry we introduce "SWIG_PL_PERL" and
"SWIG_RB_RUBY" for the Perl and Ruby SWIG bindings, and options
to specify them.

* configure.ac ():
  - Use variables "SWIG_PL_PERL", "SWIG_PY_PYTHON", "SWIG_RB_RUBY" for
    targets of the SWIG Perl, Python, Ruby bindings instead of "PERL",
    "PYTHON", "RUBY".
  - Introduce --with-swig-perl, --with-swig-python, --with-swig-ruby
    options for setting variables "SWIG_PL_PERL", "SWIG_PY_PYTHON",
    "SWIG_RB_RUBY".

* Makefile.in, build/ac-macros/swig.m4 ():
  Use variables "SWIG_PL_PERL", "SWIG_PY_PYTHON", "SWIG_RB_RUBY" for
  targets of the SWIG Perl, Python, Ruby bindings instead of "PERL",
  "PYTHON", "RUBY".

* subversion/bindings/swig/INSTALL
  (BUILDING SWIG BINDINGS FOR SVN ON UNIX step 2):
  - Describe how to specify the path to the target language interpreters
    with the new options, instead of using environment variables.
  - Fix the name of glue libraries.

Review by: danielsh