Don't set CMAKE_SYSTEM_NAME when using the Visual Studio generator

Setting CMAKE_SYSTEM_NAME and CMAKE_SYSTEM_PROCESSOR is often enough for
CMake to handle cross-compilation even without a toolchain file, but it
gets in the way of generators that handle cross-compilation on their own
(in Visual Studio's case, using the -T "toolset" option).

.github/workflows/main.yml

@@ -105,3 +105,22 @@ jobs:
       env:
         # If this isn't specified the default is iOS 7, for which zlib-ng will not compile due to the lack of thread-local storage.
         IPHONEOS_DEPLOYMENT_TARGET: 16
+
+  win_cross_compile_test:
+    name: Test Cross Compile - ${{ matrix.platform.target }}
+    needs: [ test ]
+    runs-on: windows-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        platform:
+          - target: aarch64-pc-windows-msvc
+    steps:
+    - uses: actions/checkout@v3
+    - uses: actions-rs/toolchain@v1
+      with:
+        toolchain: stable
+        target: ${{ matrix.platform.target }}
+    - name: build
+      run: cargo build -vv --target ${{ matrix.platform.target }}
+      working-directory: test-crate

src/lib.rs

@@ -438,6 +438,13 @@ impl Config {
         };
         let host = self.host.clone().unwrap_or_else(|| getenv_unwrap("HOST"));
 
+        let generator = self
+            .generator
+            .clone()
+            .or_else(|| self.getenv_target_os("CMAKE_GENERATOR"));
+
+        let msvc = target.contains("msvc");
+
         // Some decisions later on are made if CMAKE_TOOLCHAIN_FILE is defined,
         // so we need to read it from the environment variables from the beginning.
         if !self.defined("CMAKE_TOOLCHAIN_FILE") {
@@ -448,7 +455,14 @@ impl Config {
                     if !self.defined("CMAKE_SYSTEM_NAME") {
                         self.define("CMAKE_SYSTEM_NAME", "Generic");
                     }
-                } else if target != host && !self.defined("CMAKE_SYSTEM_NAME") {
+                } else if target != host
+                    && !self.defined("CMAKE_SYSTEM_NAME")
+                    && !(msvc
+                        && self
+                            .generator
+                            .as_deref()
+                            .map_or(true, |g| g.to_string_lossy().starts_with("Visual Studio")))
+                {
                     // Set CMAKE_SYSTEM_NAME and CMAKE_SYSTEM_PROCESSOR when cross compiling
                     let os = getenv_unwrap("CARGO_CFG_TARGET_OS");
                     let arch = getenv_unwrap("CARGO_CFG_TARGET_ARCH");
@@ -495,12 +509,6 @@ impl Config {
             }
         }
 
-        let generator = self
-            .generator
-            .clone()
-            .or_else(|| self.getenv_target_os("CMAKE_GENERATOR"));
-
-        let msvc = target.contains("msvc");
         let ndk = self.uses_android_ndk();
         let mut c_cfg = self.c_cfg.clone().unwrap_or_default();
         c_cfg