Combine TCL_SHLIB_LD_EXTRAS+TK_SHLIB_LD_EXTRAS to a single SHLIB_LD_LIBS usable for both Tcl and Tk.
Combine TCL_SHLIB_LD_EXTRAS+TK_SHLIB_LD_EXTRAS (for Cygwin and FreeBSD) to a single SHLIB_LD_LIBS usable for both Tcl and Tk