Build system improvements
Includes commits cherry-picked from the following upstream PRs:
- bitcoin/bitcoin#6978
- Only the first commit (second is for QT)
- bitcoin/bitcoin#7059
- bitcoin/bitcoin#7603
- Only the first commit (without the `BITCOIN_QT_BIN` variable; the rest are for QT)
- bitcoin/bitcoin#7954
- bitcoin/bitcoin#8314
- Only the second commit (first is for QT)
- bitcoin/bitcoin#8504
- Only the first commit (second was undoing something we didn't have)
- bitcoin/bitcoin#8520
- bitcoin/bitcoin#8563
- bitcoin/bitcoin#8249
- bitcoin/bitcoin#9156
- bitcoin/bitcoin#9831
- bitcoin/bitcoin#9789
- bitcoin/bitcoin#10766
Part of #2074.