xdvik: Fix setsid-after-vfork configure test for C99 compatibility

[Florian Weimer]

Building texlive 20220321 with an instrumented compiler revealed that
xdvik has a C99 compatibility issue.  This is similar to the issues
previously reported by Sam/Gentoo and fixed in r64940 and r64953.  I
don't know why this hasn't shown up in Gentoo testing; I do expect that
the result of this configure check changes on GNU/Linux with future
compilers.

Thanks,
Florian
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