meson: handle missing xkbcomp.pc better

Jon Turney requested to merge jturney/xserver:master into master

Applying get_pkgconfig_variable() to a not-found dependency was always documented as an error, but meson 0.49 now actually raises an error[1]: ERROR: 'xkbcomp' is not a pkgconfig dependency

Check xkbcomp_dep is a suitable dependency type before applying get_pkgconfig_variable() to it.

[1] but this is more by accident than design (see the discusssion at [2] et seq.), so who knows where things will come to rest...


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