Provide full Tcl patchlevel to tcl.pc and move private libs to "Libs.private".
Provide full Tcl patchlevel to tcl.pc and move private libs to "Libs.private". 
Record the fact that Tcl 8.6 requires Zlib 1.2.3 as a minimum. 
Supply both the shared library and the stub library to "Libs": Only one of them will actually be linked in, depending on whether -DUSE_TCL_STUBS is defined or not.