Use RISCV_WITH_RVV as the env var, taking ON/OFF values

Instead of `RVV_OFF`.

This checks if the value appears to match any of several hard-coded
truthy or falsy values. Typically the literal values `ON` or `OFF`
would be used, as is typically done when passing boolean `-D...`
options to `cmake` on the command-line. It is typically more useful
to pass `OFF` here than `ON`, becuase the default is `ON`, as noted
in the linked section of the zlib-ng readme.

At least for now, empty, blank, and otherwise unrecognized values
are simply ignored. Recognized values are always "normalized" as
either `ON` (if truthy) or `OFF` (if falsy).

Author: EliahKagan

zng/cmake.rs

@@ -1,4 +1,5 @@
 use std::env;
+use std::ffi::OsStr;
 
 pub fn build_zlib_ng(target: &str, compat: bool) {
     let mut cmake = cmake::Config::new("src/zlib-ng");
@@ -14,8 +15,25 @@ pub fn build_zlib_ng(target: &str, compat: bool) {
             .define("WITH_DFLTCC_INFLATE", "1")
             .cflag("-DDFLTCC_LEVEL_MASK=0x7e");
     }
-    if target.contains("riscv64") && env::var_os("RVV_OFF").is_some() {
-        cmake.define("WITH_RVV", "OFF");
+    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
+        match env::var_os("RISCV_WITH_RVV")
+            .map(OsStr::to_str)
+            .map(str::trim)
+            .map(str::to_uppercase)
+            .map(Into::into)
+        {
+            Some("OFF" | "NO" | "FALSE" | "0") => {
+                // Force RVV off. This turns off RVV entirely, as well as the runtime check for it.
+                cmake.define("WITH_RVV", "OFF");
+            }
+            Some("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");