build: Qt 5.15.2

[barton2526]

Opening this for visibility. Apart from it probably being time to use a newer version of Qt for releases, an update is required so that we are using a version which has a C++17 mode.

The biggest change here is that Qt no longer has internal xcb libs, so we need to build the requirements ourselves.

Ref: bitcoin/bitcoin#19716
Ref: bitcoin/bitcoin#23489

• build, qt: Ditch no_sdk_version_check.patch
  Ref: build, qt: Ditch no_sdk_version_check.patch bitcoin/bitcoin#23618

• build: Post-pr23489 small cleanups
  Ref: build: Post-pr23489 small cleanups bitcoin/bitcoin#23675

• build, qt, macOS: Don't hard-code x86_64 as the arch when using qmake
  Ref: build, qt, macOS: Don't hard-code x86_64 as the arch when using qmake bitcoin/bitcoin#23583

[barton2526]

CI is failing on Windows. Any assistance in debugging would be appreciated.

Here is the error I see locally:

In file included from rhi/qrhi.cpp:49:0:
rhi/qrhid3d11_p_p.h:57:10: fatal error: dxgi1_3.h: No such file or directory
 #include <dxgi1_3.h>
          ^~~~~~~~~~~
compilation terminated.
Makefile.Release:71665: recipe for target '.obj/release/qrhi.o' failed```

[barton2526]

Here is a patch we could use
bitcoin/bitcoin#19716 (comment)

[barton2526]

This currently compiles for me on Windows 10 WSL cross-compile (Ubuntu Bionic)

[jamescowens]

@barton2526 please rebase this to development now that I merged #2484.

[barton2526]

Moved to #2486