Revision 1884610

Distinguish configure scripts on release mode and non release mode.

Although makefiles in Subversion's release tarball do not support generating
SWIG language bindings C source files using swig, the configure scripts
shipped with the release tarball had an option to specify how to find
SWIG executable, and checked it.  To avoid this, we introduce "release mode"
to the configure script and hide the option and code to check a SWIG
executable on it.  

* . (svn:ignore):
  Ignore aclocal.m4.
* Makefile.in (local-extraclean):
  Clean up aclocal.m4.
* aclocal.m4.in ():
  Renamed from aclocal.m4
* autogen.sh ():
  Write an macro "SVN_RELEASE_MODE" definition to aclocal.m4 if --release
  is specfied on the command line.
* build/ac-macros/swig.m4
  (): Also mension about Perl and Ruby.
  (SVN_CHECK_SWIG):
   - Hide --with-swig option in release mode.
   - Check SWIG executable only if non release mode and at least one of 
     SWIG bindings is specfied to be built.
  (SVN_FIND_SWIG):
    Move checks for each bindings into new macro SVN_DETERMINE_SWIG_OPTS.
  (SVN_DETERMINE_SWIG_OPTS): New macro split from SVN_FIND_SWIG. 
  - When not in release mode, warn if Perl/Python/Ruby interpreter is set but
   SWIG is not found. Also do not build them with make swig-pl/make swig-py/
   make swig-rb in this case.
  - Check swig version only when it is needed and when not in release mode.
* configure.ac ():
  - Tweak help string for --with-swig-perl, --with-swig-python,
    --with-swig-ruby. 
  - Warn if --with-swig-perl is not specified but variable 'PERL' is set.
  - Warn if --with-swig-ruby is not specified but variable 'RUBY' is set,
    even the value is not 'no' nor 'none'.
* subversion/bindings/swig/INSTALL (Step 2):
  Mention that configure and makefiles in the release tarball don't
  support generating SWIG bindings C source files.

Review by: brane
           (commit message, help string, and the structure of
            SVN_CHECK_SWIG macro)