Add the --tclConfig.sh FILE flag to buildtclext.tcl to eliminate the discrepancy...

Add the --tclConfig.sh FILE flag to buildtclext.tcl to eliminate the discrepancy between the configure-time-detected tclConfig.sh and the one auto-detected by the tclextension family of makefile targets. Add the tclextension-all target to run all of the various tclextension-... targets in their natural order to facilitate testing changes like this one. Update the --help text to note that --with-tclsh should only be used if there's a specific need, and that --with-tcl is generall preferred. This is in response to [https://github.com/termux/termux-packages/issues/23268 | Termux ticket 23268].

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