dependencies: reject Qt6 pkg-config dependency if libexecdir is not known
dependencies: reject Qt6 pkg-config dependency if libexecdir is not known

This is needed for 6.1 support as noted in commit
a606ce22eb0dd05eef56384d59954edad7da131d but at the time there was no
pkg-config detection method at all. The stub code for Qt6 didn't support
libexecdir at all, and the pkg-config files added to newer versions of
Qt6 didn't export that information either until a further bugfix release.

Enforce that this information is required.

mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,pexip/meson,pexip/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson