[depends] Backport various changes to Qt build

Summary:

• [Depends] Fix Qt build with Xcode 9.2 (PR11995)
• depends: Fix Qt build with XCode 9.3 (PR12946)
• depends: Update Qt download url (PR13544)

Test Plan:

make -C ../depends build-all

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1811