cmake: Avoid hardcoding Qt's major version in Find module

This change facilitates future migration to Qt 6.

cmake/module/FindQt.cmake

@@ -0,0 +1,66 @@
+# Copyright (c) 2024-present The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or https://opensource.org/license/mit/.
+
+#[=======================================================================[
+FindQt
+------
+
+Finds the Qt headers and libraries.
+
+This is a wrapper around find_package() command that:
+ - facilitates searching in various build environments
+ - prints a standard log message
+
+#]=======================================================================]
+
+set(_qt_homebrew_prefix)
+if(CMAKE_HOST_APPLE)
+  find_program(HOMEBREW_EXECUTABLE brew)
+  if(HOMEBREW_EXECUTABLE)
+    execute_process(
+      COMMAND ${HOMEBREW_EXECUTABLE} --prefix qt@${Qt_FIND_VERSION_MAJOR}
+      OUTPUT_VARIABLE _qt_homebrew_prefix
+      ERROR_QUIET
+      OUTPUT_STRIP_TRAILING_WHITESPACE
+    )
+  endif()
+endif()
+
+# Save CMAKE_FIND_ROOT_PATH_MODE_LIBRARY state.
+unset(_qt_find_root_path_mode_library_saved)
+if(DEFINED CMAKE_FIND_ROOT_PATH_MODE_LIBRARY)
+  set(_qt_find_root_path_mode_library_saved ${CMAKE_FIND_ROOT_PATH_MODE_LIBRARY})
+endif()
+
+# The Qt config files internally use find_library() calls for all
+# dependencies to ensure their availability. In turn, the find_library()
+# inspects the well-known locations on the file system; therefore, it must
+# be able to find platform-specific system libraries, for example:
+# /usr/x86_64-w64-mingw32/lib/libm.a or /usr/arm-linux-gnueabihf/lib/libm.a.
+set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY BOTH)
+
+find_package(Qt${Qt_FIND_VERSION_MAJOR} ${Qt_FIND_VERSION}
+  COMPONENTS ${Qt_FIND_COMPONENTS}
+  HINTS ${_qt_homebrew_prefix}
+  PATH_SUFFIXES Qt${Qt_FIND_VERSION_MAJOR}  # Required on OpenBSD systems.
+)
+unset(_qt_homebrew_prefix)
+
+# Restore CMAKE_FIND_ROOT_PATH_MODE_LIBRARY state.
+if(DEFINED _qt_find_root_path_mode_library_saved)
+  set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ${_qt_find_root_path_mode_library_saved})
+  unset(_qt_find_root_path_mode_library_saved)
+else()
+  unset(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY)
+endif()
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(Qt
+  REQUIRED_VARS Qt${Qt_FIND_VERSION_MAJOR}_DIR
+  VERSION_VAR Qt${Qt_FIND_VERSION_MAJOR}_VERSION
+)
+
+foreach(component IN LISTS Qt_FIND_COMPONENTS ITEMS "")
+  mark_as_advanced(Qt${Qt_FIND_VERSION_MAJOR}${component}_DIR)
+endforeach()