Recognize RISCV_WITH_RVV env var on RISC-V to set WITH_RVV cmake var

zng/cmake.rs

@@ -14,6 +14,27 @@ pub fn build_zlib_ng(target: &str, compat: bool) {
             .define("WITH_DFLTCC_INFLATE", "1")
             .cflag("-DDFLTCC_LEVEL_MASK=0x7e");
     }
+    if target.contains("riscv") {
+        // Check if we should pass on an explicit boolean value of the WITH_RVV build option.
+        // See: https://github.com/zlib-ng/zlib-ng?tab=readme-ov-file#advanced-build-options
+        if let Ok(value) = env::var("RISCV_WITH_RVV") {
+            match value.trim().to_uppercase().as_str() {
+                "OFF" | "NO" | "FALSE" | "0" => {
+                    // Force RVV off. This turns off RVV entirely, as well as the runtime check for it.
+                    // This is not usually necessary, but can be useful for building binaries portable
+                    // to systems that do not support RVV but where auto-detection fails to identify
+                    // this (as in https://github.com/zlib-ng/zlib-ng/issues/1705).
+                    cmake.define("WITH_RVV", "OFF");
+                }
+                "ON" | "YES" | "TRUE" | "1" => {
+                    // Try to use RVV, but still don't do so if a runtime check finds it unavailable.
+                    // This has the same effect as omitting WITH_RVV, unless it has already been set.
+                    cmake.define("WITH_RVV", "ON");
+                }
+                _ => {}
+            }
+        }
+    }
     if target == "i686-pc-windows-msvc" {
         cmake.define("CMAKE_GENERATOR_PLATFORM", "Win32");
     }