ci: same CMake build on Windows UCRT64

Linux alone would leave the platform where portability actually breaks
untested -- and it is the one the dead qmake workflow was aimed at.

Same standalone build as the Ubuntu job, same RS_* option set, so a failure on
one and not the other means a portability problem rather than a difference in
what was configured. What differs is what has to: pacman instead of apt, and the
two -include flags the super-project's Windows job also carries, since librnp
uses strlen and the fixed width integer types without including <cstring> and
<cstdint>, which this toolchain refuses.

No Qt packages here: this repository builds no GUI.

macOS is deliberately left out for now. Its counterpart in the super-project is
mostly Homebrew flag harvesting for Qt5, ffmpeg and speex -- GUI and VOIP
dependencies that libretroshare does not use -- so it would be a rewrite rather
than a transposition, with little to catch that Linux does not.

.github/workflows/windows-cmake.yml

@@ -0,0 +1,82 @@
+# Windows (MSYS2 / UCRT64) CMake CI for libretroshare on its own.
+#
+# Same standalone build as ubuntu-cmake.yml: libretroshare has its own top level
+# CMakeLists.txt and pulls rnp, OpenPGP-SDK, BitDHT, rapidjson, restbed and
+# udp-discovery-cpp through FetchContent, so no super-project and no submodules
+# are involved. Only the MSYS2 packages below are installed.
+#
+# Windows earns its own job rather than a matrix entry: it is where portability
+# breaks first, its dependencies come from pacman rather than apt, and it needs
+# the two -include flags below, which the super-project's Windows job also
+# carries -- librnp uses strlen and the fixed width integer types without
+# including <cstring> and <cstdint>, which this toolchain refuses.
+#
+# No Qt packages: this repository builds no GUI.
+#
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+name: Windows UCRT64 CMake C/C++ CI
+
+on:
+  push:
+    paths-ignore:
+    - '**/*.md'
+  pull_request:
+    paths-ignore:
+    - '**/*.md'
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+    name: Windows UCRT64 / CMake
+    runs-on: windows-latest
+    defaults:
+      run:
+        shell: msys2 {0}
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          persist-credentials: false
+
+      - name: Setup MSYS2 (UCRT64)
+        uses: msys2/setup-msys2@40677d36a502eb2cf0fb808cc9dec31bf6152638 # v2.28.0
+        with:
+          msystem: UCRT64
+          update: true
+          install: >-
+            base-devel
+            git
+            mingw-w64-ucrt-x86_64-toolchain
+            mingw-w64-ucrt-x86_64-cmake
+            mingw-w64-ucrt-x86_64-ninja
+            mingw-w64-ucrt-x86_64-python
+            mingw-w64-ucrt-x86_64-doxygen
+            mingw-w64-ucrt-x86_64-openssl
+            mingw-w64-ucrt-x86_64-sqlcipher
+            mingw-w64-ucrt-x86_64-miniupnpc
+            mingw-w64-ucrt-x86_64-json-c
+            mingw-w64-ucrt-x86_64-bzip2
+            mingw-w64-ucrt-x86_64-zlib
+
+      - name: CMake configure
+        run: >
+          cmake -G Ninja -B Build-cmake -S .
+          -DCMAKE_POLICY_VERSION_MINIMUM=3.5
+          -DCMAKE_BUILD_TYPE=Release
+          -DRS_RNPLIB=ON -DRS_JSON_API=ON -DRS_WEBUI=OFF
+          -DRS_SQLCIPHER=ON -DRS_BITDHT=ON
+          -DRS_BRODCAST_DISCOVERY=ON -DRS_MINIUPNPC=ON
+          -DRS_FORUM_DEEP_INDEX=OFF
+          -DCMAKE_C_FLAGS="-include string.h -include stdint.h"
+          -DCMAKE_CXX_FLAGS="-include cstring -include cstdint -Wno-template-body"
+
+      - name: CMake build
+        run: cmake --build Build-cmake -j3