Build issues when packaging version 3.3.0

[ColinDuquesnoy]

Happens on openSUSE Build System as well as with PKGBUILD:

• on openSUSE: failed to build QmlImports target because of link error with QtCore -> SOLUTION: link with Qt5::Core
• on Xubuntu17.10: check_qml_syntax fail because of missing Qt installation environment variable -> SOLUTION: turn QML check OFF by default
• pkgbuild issue of in source tree builds: update_imports target is always run which cause cmake to reload and try to compile auto generated moc files. SOLUTION: don't run update_imports automatically. This target is for developpers only when they add/remove qml/js

[ColinDuquesnoy]

Fixed in commit d969abc