fix: embed C compiler path at build time for self-contained binary

build.rs captures the absolute path to cc/gcc/clang during compilation
and bakes it into the binary. On Nix systems this embeds the full
/nix/store path so `lux compile` works without cc on PATH.

Lookup order: $CC env var > embedded build-time path > PATH search.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/main.rs

@@ -726,6 +726,36 @@ fn collect_lux_files_nonrecursive(dir: &str, pattern: Option<&str>, files: &mut
     }
 }
 
+/// Find a C compiler. Priority: $CC env var, build-time embedded path, PATH search.
+fn find_c_compiler() -> String {
+    // 1. Explicit env var
+    if let Ok(cc) = std::env::var("CC") {
+        if !cc.is_empty() {
+            return cc;
+        }
+    }
+    // 2. Path captured at build time (e.g. absolute nix store path)
+    let built_in = env!("LUX_CC_PATH");
+    if !built_in.is_empty() && std::path::Path::new(built_in).exists() {
+        return built_in.to_string();
+    }
+    // 3. Search PATH
+    for name in &["cc", "gcc", "clang"] {
+        if let Ok(output) = std::process::Command::new("which").arg(name).output() {
+            if output.status.success() {
+                if let Ok(p) = String::from_utf8(output.stdout) {
+                    let p = p.trim();
+                    if !p.is_empty() {
+                        return p.to_string();
+                    }
+                }
+            }
+        }
+    }
+    // 4. Last resort
+    "cc".to_string()
+}
+
 fn compile_to_c(path: &str, output_path: Option<&str>, run_after: bool, emit_c: bool) {
     use codegen::c_backend::CBackend;
     use modules::ModuleLoader;
@@ -817,8 +847,8 @@ fn compile_to_c(path: &str, output_path: Option<&str>, run_after: bool, emit_c:
         std::process::exit(1);
     }
 
-    // Find C compiler
-    let cc = std::env::var("CC").unwrap_or_else(|_| "cc".to_string());
+    // Find C compiler: $CC env var > embedded build-time path > PATH search
+    let cc = find_c_compiler();
 
     let compile_result = Command::new(&cc)
         .args(["-O2", "-o"])