system, not Tcl-provided one.

Make sure export file, if any, is cleaned.

Tcl configuration is now read directly in configure and recorded in
Makefile.global.  This eliminates some duplicate efforts and allows
for easier hand-editing of the results, if necessary.

calls with new or now-built-in versions.  Make sure that all
calls to AC_DEFINE have a third argument, for possible use of
autoheader in the future.

add --without-tk option to disable Tk.  We don't need the AC_PATH_XTRA
test because tkConfig.sh already contains all the information about how to
compile and link with X.  Also make sure that libpq is up to date for
libpgtcl.  Remove executable bits from pgaccess.sh, but add it to pgaccess.