Basic idea for rvv-off feature

So far, this only attempts to have an effect when `cmake` is used,
even though I documented it to be broader than that.

In addition to testing this feature, it should either:

- Be broadened to cover the non-`cmake` case (which I am unsure how
  to do, because which `-march` selection should be passed to `cc`
  varies according to other features too, which I believe `cmake`
  takes care of detecting), or

- Have its documentation narrowed to say it only applies when
  `cmake` is used.

Author: EliahKagan

Cargo-zng.toml

@@ -38,5 +38,11 @@ libc = "0.2.43"
 [build-dependencies]
 cmake = "0.1.50"
 
+[features]
+# This feature builds riscv64 binaries portable to machines without RVV, even if the build machine
+# is riscv64 and detected to support RVV. This is useful for building a more portable binary, or on
+# systems where auto-detection is incorrect (as in https://github.com/zlib-ng/zlib-ng/issues/1705).
+rvv-off = []
+
 [lints.rust]
 unexpected_cfgs = { level = "warn", check-cfg = ['cfg(zng)'] }

Cargo.toml

@@ -89,6 +89,12 @@ zlib-ng = ["libc", "cmake"]
 # or be completely removed in the future
 zlib-ng-no-cmake-experimental-community-maintained = ["libc"]
 stock-zlib = []
+# This feature only affects zlib-ng and is only relevant to riscv64 targets. It
+# builds binaries portable to machines without RVV, even if the build machine
+# is riscv64 and detected to support RVV. This is useful for building a more
+# portable binary, or on systems where auto-detection is incorrect
+# (as in https://github.com/zlib-ng/zlib-ng/issues/1705).
+rvv-off = []
 # Deprecated: the assembly routines are outdated, and either reduce performance
 # or cause segfaults.
 asm = []

zng/cmake.rs

@@ -14,6 +14,9 @@ pub fn build_zlib_ng(target: &str, compat: bool) {
             .define("WITH_DFLTCC_INFLATE", "1")
             .cflag("-DDFLTCC_LEVEL_MASK=0x7e");
     }
+    if cfg!(feature = "rvv-off") && target.contains("riscv64") {
+        cmake.define("WITH_RVV", "OFF");
+    }
     if target == "i686-pc-windows-msvc" {
         cmake.define("CMAKE_GENERATOR_PLATFORM", "Win32");
     }