meson: handle missing xkbcomp.pc better
Applying get_pkgconfig_variable() to a not-found dependency was always documented as an error, but meson 0.49 now actually raises an error:
meson.build:110:4: ERROR: 'xkbcomp' is not a pkgconfig dependency
Check xkbcomp_dep is a suitable dependency type before applying get_pkgconfig_variable() to it.
 but this is more by accident than design (see the discusssion at  et seq.), so who knows where things will come to rest...