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.