[tlbuild] A minor documentation error in TL2019 top-level configure

[Nelson H. F. Beebe]

A local user reports this problem:

>> ...
>> The
>> 
>> 	configure --help
>> 
>> command (on the top level of the source tree) gives among other things
>> 
>> --with-xdvi-x-toolkit=KIT
>>                            Use toolkit KIT (motif/xaw/xaw3d/neXtaw) for xdvi
>>                            [default: Motif if available, else Xaw]
>> 
>> The last statement is not true. On Solaris, motif is available, but
>> the default build is with Xaw. To do the motif build, one has to add it
>> explicitly.
>> ...


The texlive/2019/bin/x86_64-linux/xdvi script runs the executable
texlive/2019/bin/x86_64-linux/xdvi-xaw; the same is true for the
x86_64-solaris binaries.

Thus, the bracketed help message should perhaps be

	[default: Xaw if available, else Motif]

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