Define FXX_GCC in fsr-sys build script at runtime based on CARGO_CFG_…

…UNIX (#10)

A compile-time check for the target_os in a build script will describe
the build host when cross-compiling, not the final target.

This fixes being able to cross-compile the fsr-sys crate for Android on
Windows by checking for the `CARGO_CFG_UNIX` environment variable at
runtime instead.

fsr-sys/build/build.rs

@@ -18,8 +18,9 @@ fn build_fsr(api_dir: &str, _vk_include_dir: &str) {
         .cpp(true)
         .define("DYNAMIC_LINK_VULKAN", "1");
 
-    #[cfg(not(target_os = "windows"))]
-    build.define("FFX_GCC", "1").std("c++2a");
+    if std::env::var("CARGO_CFG_UNIX").is_ok() {
+        build.define("FFX_GCC", "1").std("c++2a");
+    }
 
     #[cfg(feature = "vulkan")]
     build