GHA Windows: update handling of vcpkg integration

.github/workflows/actions.yml

@@ -488,13 +488,11 @@ jobs:
         shell: bash
         run: |
           export PATH=$Qt5_DIR/bin:$PATH
+          export VCPKG_ROOT=$VCPKG_INSTALLATION_ROOT
 
           mkdir $BUILD_PATH && cd $BUILD_PATH
 
-          EXTRA_CMAKE_FLAGS=
-          if [[ -n "${{ matrix.vcpkg-triplet }}" ]]; then EXTRA_CMAKE_FLAGS="-D CMAKE_TOOLCHAIN_FILE=$VCPKG_INSTALLATION_ROOT/scripts/buildsystems/vcpkg.cmake $EXTRA_CMAKE_FLAGS"; fi
-
-          cmake -G "${{ matrix.cmake-generator }}" -A "${{ matrix.cmake-arch }}" -D CMAKE_PREFIX_PATH=$Qt5_DIR -D SUPERNOVA=ON -D SC_USE_QTWEBENGINE=${{ matrix.use-qtwebengine }} -D SC_ABLETON_LINK=${{ matrix.ableton-link }} -D CMAKE_BUILD_TYPE=Release $EXTRA_CMAKE_FLAGS .. # build type is specified here for MinGW build
+          cmake -G "${{ matrix.cmake-generator }}" -A "${{ matrix.cmake-arch }}" -D CMAKE_PREFIX_PATH="$Qt5_DIR" -D SUPERNOVA=ON -D SC_USE_QTWEBENGINE=${{ matrix.use-qtwebengine }} -D SC_ABLETON_LINK=${{ matrix.ableton-link }} -D CMAKE_BUILD_TYPE=Release .. # build type is specified here for MinGW build
 
       - name: build
         shell: bash

CMakeLists.txt

@@ -1,4 +1,11 @@
 cmake_minimum_required(VERSION 3.12)
+
+# pick up vcpkg installation, if VCPKG_ROOT is set
+if(DEFINED ENV{VCPKG_ROOT} AND NOT DEFINED CMAKE_TOOLCHAIN_FILE AND NOT "$ENV{VCPKG_ROOT}" STREQUAL "")
+    set(CMAKE_TOOLCHAIN_FILE "$ENV{VCPKG_ROOT}/scripts/buildsystems/vcpkg.cmake" CACHE STRING "")
+    message(STATUS "Using CMAKE_TOOLCHAIN_FILE at ${CMAKE_TOOLCHAIN_FILE}")
+endif()
+
 include(SCVersion.txt)
 # CMake versions can only consist of digits and periods, so don't use _TWEAK
 project(SuperCollider VERSION ${SC_VERSION_MAJOR}.${SC_VERSION_MINOR}.${SC_VERSION_PATCH})

README_WINDOWS.md

@@ -1209,15 +1209,16 @@ if you require an advanced configuration, and are interested in this approach.
 
 ### Readline support
 
-Previously Windows builds of SuperCollider did not support command line mode for sclang due to unavailability of the `readline` library. Currently it is possible to install `readline` using [vcpkg](https://github.com/microsoft/vcpkg). Here are steps to build SC with `readline` library (the following commands assume using MSVC):
+Previously Windows builds of SuperCollider did not support command line mode for sclang due to unavailability of the `readline` library. Currently it is possible to install `readline` using [vcpkg](https://github.com/microsoft/vcpkg). Follow these steps to build SC with the `readline` library (note that the following commands assume using MSVC):
 - install `vcpkg`, if not installed already
 - install the library using `vcpkg`:  
 `vcpkg install readline --triplet=x64-windows`
-- include `vcpkg` path in `CMAKE_PREFIX_PATH` (in addition to the Qt path defined already):  
-`SET CMAKE_PREFIX_PATH=<qt_path>;<vcpkg_root>\installed\x64-windows`
+- set `VCPKG_ROOT` environment variable to point to the root `vcpkg` directory:  
+`SET VCPKG_ROOT=c:\path\to\vcpkg`
+- readline should be picked up by CMake during the configuration stage
 
 Note: 
-- For 32-bit builds use `x86-windows` instead of `x64-windows` triplet, both when installing `readline` as well as specifying `CMAKE_PREFIX_PATH`
+- For 32-bit builds use `x86-windows` instead of `x64-windows` triplet when installing `readline`
 - At the time of writing this, `readline` would not build using a triplet for MinGW
 
 Known issues