Expose the GSUSER_SUBDIR preprocessor symbol to Rust.

This symbol's value must be known to port `wmkdirhier` and `wrmdirhier` from
`proplist.c` to Rust.

This change introduces a basic C library under wutil-rs that is linked into the
Rust code to expose preprocessor symbols and other Autotools configuration
decisions to Rust. See the rust rewrite notes at the top of
`wutil-rs/src/defines.rs` for further thoughts.

wutil-rs/Cargo.toml

@@ -5,3 +5,6 @@ edition = "2024"
 
 [lib]
 crate-type = ["staticlib"]
+
+[build-dependencies]
+cc = "1.0"

wutil-rs/build.rs

@@ -0,0 +1,8 @@
+use cc;
+
+fn main() {
+    cc::Build::new()
+        .file("src/defines.c")
+        .compile("defines");
+    println!("cargo::rerun-if-changed=src/defines.c");
+}

wutil-rs/src/defines.c

@@ -0,0 +1,5 @@
+#include "../../config-paths.h"
+
+const char *get_GSUSER_SUBDIR() {
+  return GSUSER_SUBDIR;
+}

wutil-rs/src/defines.rs

@@ -0,0 +1,45 @@
+//! Lookup functions for preprocessor symbols.
+//!
+//! Functions in this module may be called to get the value of various
+//! preprocessor symbols that are available on the C side of things.
+//!
+//! ## Rust rewrite notes
+//!
+//! Until we move away from autootols entirely, we might be stuck with this as
+//! along as it makes sense to keep the configure script as the main entrypoint
+//! for compile-time configuration.
+
+use std::ffi::{c_char, CStr};
+
+// Functions defined in src/defines.c.
+unsafe extern "C" {
+    fn get_GSUSER_SUBDIR() -> *const c_char;
+}
+
+/// Returns the value of the GSUSER_SUBDIR preprocessor symbol defined at
+/// Autotools configuration time prior to compilation. This is the final
+/// component of the root path where user data files are stored (usually
+/// `GNUstep`, as in `$HOME/GNUstep`). Returns `None` if this value cannot be
+/// determined or is empty.
+pub fn gsuser_subdir() -> Option<String> {
+    let s = unsafe { get_GSUSER_SUBDIR() };
+    if s.is_null() {
+        return None;
+    }
+    let s = unsafe { CStr::from_ptr(s) };
+    if s.is_empty() {
+        return None;
+    }
+    String::from_utf8(s.to_bytes().iter().copied().collect()).ok()
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn gsuser_subdir_is_set() {
+        let s = gsuser_subdir().unwrap();
+        assert!(!s.is_empty());
+    }
+}

wutil-rs/src/lib.rs

@@ -1,4 +1,5 @@
 pub mod array;
 pub mod data;
+pub mod defines;
 pub mod find_file;
 pub mod memory;