Port UVAR_FILE_SET_MTIME_HACK

Notably this exposes config.h to Rust (for UVAR_FILE_SET_MTIME_HACK).
In future we should move the CMake checks into build.rs so we can potentially
get rid of CMake.

fish-rust/build.rs

@@ -12,6 +12,11 @@ fn main() {
 
     cc::Build::new()
         .file("fish-rust/src/compat.c")
+        .include(
+            &std::env::var("FISH_BUILD_DIR")
+                // Add our default to potentially help tools that don't go through CMake.
+                .unwrap_or("./build".into()),
+        )
         .compile("libcompat.a");
 
     let rust_dir = env!("CARGO_MANIFEST_DIR");

fish-rust/src/compat.c

@@ -1,3 +1,6 @@
+#include "config.h"
+
+#include <stdbool.h>
 #include <stdint.h>
 #include <stdlib.h>
 #include <term.h>
@@ -47,3 +50,12 @@ uint64_t C_MNT_LOCAL() {
     return 0;
 #endif
 }
+
+static const bool uvar_file_set_mtime_hack =
+#ifdef UVAR_FILE_SET_MTIME_HACK
+    true;
+#else
+    false;
+#endif
+#undef UVAR_FILE_SET_MTIME_HACK
+bool UVAR_FILE_SET_MTIME_HACK() { return uvar_file_set_mtime_hack; }