gh-103839: Allow building Tkinter against Tcl 8.7 without external libtommath

[chrstphrchvz]

• Issue: TIP 538 prevents building Tkinter against Tcl 8.7 built with bundled libtommath #103839

[chrstphrchvz]

Although checking for TCL_WITH_EXTERNAL_TOMMATH before defining TCL_NO_TOMMATH_H seems correct to me, I now notice some inadequacies in how Tkinter is gathering Tcl build flags and/or how Tcl is reporting them. By default, Tkinter tries to use pkg-config to obtain Cflags from …/lib/pkgconfig/tcl.pc. But Tcl only records the -DTCL_WITH_EXTERNAL_TOMMATH flag in the TCL_DEFS line of tclConfig.sh (which is where Tcl/Tk software conventionally would check); most of the information in tclConfig.sh is not present in tcl.pc. I will likely ask Tcl if this can be improved.

[ned-deily]

As a step toward future full support of Tk 8.7, this seems harmless enough. I don't think it should be backported until and unless we decide to backport full 8.7 support with subsequent PRs.