Merge pull request #1580 from ispyisail/doc/install-msys2-kf6

Document KDE Frameworks for Windows builds through MSYS2 UCRT64

INSTALL.md

@@ -227,9 +227,11 @@ for why this matters).
 `QtPdf` is available as an optional component in the Qt Online Installer
 (look for "Qt PDF" under the Qt 6.4+ node); if you skip it, PDF page import
 is silently disabled exactly as on the other platforms. KDE Frameworks are
-not prebuilt for Windows by any of the routes below, so Windows builds
-normally use `-DBUILD_WITH_KF=OFF` (see the
-[autosave caveat](#9-building-without-kde-frameworks)).
+prebuilt for Windows only in MSYS2's UCRT64 and CLANG64 environments (the
+UCRT64 route below, which is also what the official Windows build uses).
+With MSVC or any other MinGW, use `-DBUILD_WITH_KF=OFF` (see the
+[autosave caveat](#9-building-without-kde-frameworks)); otherwise the build
+downloads and compiles KDE Frameworks itself, which is slow.
 
 ### MSVC (Visual Studio 2019 / 2022)
 
@@ -259,25 +261,35 @@ Two common ways to get a MinGW toolchain that matches a Qt6 MinGW kit: Qt's
 own bundled MinGW (installed alongside a `mingw_64` Qt kit via the Qt
 Online Installer), or [MSYS2](https://www.msys2.org).
 
-Using MSYS2, from a **MINGW64** shell:
+Using MSYS2, from a **UCRT64** shell (the same packages and options as
+`.github/workflows/windows-build.yml`):
 ```sh
 pacman -S --needed \
-    mingw-w64-x86_64-toolchain \
-    mingw-w64-x86_64-qt6-base mingw-w64-x86_64-qt6-tools \
-    mingw-w64-x86_64-cmake mingw-w64-x86_64-ninja \
-    mingw-w64-x86_64-sqlite3
+    mingw-w64-ucrt-x86_64-toolchain \
+    mingw-w64-ucrt-x86_64-qt6-base mingw-w64-ucrt-x86_64-qt6-svg \
+    mingw-w64-ucrt-x86_64-qt6-tools mingw-w64-ucrt-x86_64-qt6-translations \
+    mingw-w64-ucrt-x86_64-cmake mingw-w64-ucrt-x86_64-ninja \
+    mingw-w64-ucrt-x86_64-pkg-config mingw-w64-ucrt-x86_64-sqlite3 \
+    mingw-w64-ucrt-x86_64-kcoreaddons mingw-w64-ucrt-x86_64-kwidgetsaddons \
+    mingw-w64-ucrt-x86_64-extra-cmake-modules
 
 mkdir build && cd build
-cmake .. -G Ninja -DBUILD_WITH_KF=OFF -DCMAKE_BUILD_TYPE=Release
+cmake .. -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_PREFIX_PATH=/ucrt64 \
+    -DBUILD_WITH_KF=ON -DBUILD_KF=OFF \
+    -DCMAKE_POLICY_VERSION_MINIMUM=3.5 \
+    -DSQLite3_INCLUDE_DIR=/ucrt64/include \
+    -DSQLite3_LIBRARY=/ucrt64/lib/libsqlite3.dll.a
 cmake --build .
 ```
 
-If `libbacktrace` is installed in your MSYS2 environment, pass the following
+`-DBUILD_KF=OFF` makes the build use the KDE Frameworks packages just
+installed instead of downloading and compiling its own.
+
+If `libbacktrace` is installed in your MSYS2 environment, add the following
 cached CMake variables when configuring. They are only necessary in that
 case, because `FindBacktrace` needs them to locate the library:
 
 ```sh
-cmake .. -G Ninja -DBUILD_WITH_KF=OFF -DCMAKE_BUILD_TYPE=Release \
     -DBacktrace_INCLUDE_DIR=/c/msys64/clang64/include \
     -DBacktrace_LIBRARY=/c/msys64/clang64/lib/libbacktrace.a
 ```
@@ -287,7 +299,10 @@ and `Backtrace_LIBRARY` to the `libbacktrace.a` file. The paths above are the
 usual locations for the MSYS2 `clang64` environment; adjust them if your
 installation uses a different prefix.
 
-(KF6 isn't packaged in MSYS2 either, hence `-DBUILD_WITH_KF=OFF` again.)
+KF6 is packaged for MSYS2's UCRT64 and CLANG64 environments only. In a
+**MINGW64** shell (`mingw-w64-x86_64-*` packages) it is not available: install
+the same packages without the three KDE ones and configure with
+`-DBUILD_WITH_KF=OFF` instead of `-DBUILD_WITH_KF=ON -DBUILD_KF=OFF`.
 
 Using the Qt Online Installer's bundled MinGW kit instead: point
 `CMAKE_PREFIX_PATH` at that kit (e.g. `C:\Qt\6.x.x\mingw_64`) and make sure
@@ -332,7 +347,8 @@ platforms, including FreeBSD as of this writing.
 `BUILD_WITH_KF=OFF` builds QET against a bundled fallback UI layer instead
 of KDE Frameworks (`kcoreaddons`/`kwidgetsaddons`). This is a fully
 supported configuration and is, in practice, the default on platforms where
-KF6 isn't readily packaged (macOS and Windows in particular).
+KF6 isn't readily packaged (macOS, and Windows outside MSYS2's UCRT64 and
+CLANG64 environments).
 
 Almost everything works identically either way. The one known caveat:
 **autosave behavior is not as polished without KF**, since QET relies on