TCL/TK configuration clean-up patches

The attached patches clean-up the TCL/TK configuration as follows:

1.  Removed unnecessary code relating to TCL/TK configuration from configure.

2.  Change TCL/TK enabling to be dependant on the existance of an executable
    tclsh (locatable via $PATH) and the existance of tclConfig.sh and
    tkConfig.sh.

3.  The directories that are searched for the *Config.sh file is determined by
    the contents of $LIBRARY_DIRS (set by '--with-libs' or '--with-libraries')
    and the output generated by executing 'echo "puts $auto_path" | tchsh".
    [Thanks Roland!]

    Note: If TK is installed in a different location the TCL, you must use the
          --with-libs (or --with-libraries) option of configure to specify it's
          location.

4.  Added "USE_TK" to Makefile.global which is set if TK support is available
    (as determined by the existance of tkConfig.sh).  USE_TK will only be set
    true if USE_TCL is true, and TK support is available.  This will allow
    features/programs that only depend on TCL to compile and install even if
    TK support is missing.

5.  Modified the pgtclsh Makefile so that pgtclsh will compile and install even
    if TK support is missing.  pgtksh will not be built unless TK support is
    available.

NOTE: The file, bin/pgtclsh/mkMakefile.tcltkdefs.sh.in, is no longer needed and
      can be removed.

NOTE: With these changes (and earlier ones), manually setting USE_TCL and
      USE_TK in Makefile.global becomes a very bad idea and will cause the
      build to fail.

____       | Billy G. Allie    | Domain....: Bill.Allie@mug.org
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