configure: Look for Qt6 moc as "/usr/libexec/moc"
Search for Qt6 moc as /usr/libexec/moc on autotools builds.
Might push to main/S3_2 already before 3.1.0-beta1 gets released, and only wait with S3_1.
Search for Qt6 moc as /usr/libexec/moc on autotools builds.