Move posix_spawn detection to build.rs

This is *simpler*, and would have made us detect the issue with broken
includes before.

Part of #12055

build.rs

@@ -92,7 +92,15 @@ fn detect_cfgs(target: &mut Target) {
             target.has_symbol("localeconv_l")
         }),
         ("have_pipe2", &|target| target.has_symbol("pipe2")),
-        ("have_posix_spawn", &|target| target.has_header("spawn.h")),
+        ("have_posix_spawn", &|target| {
+            if target_os() == "openbsd" {
+                // OpenBSD's posix_spawn returns status 127 instead of erroring with ENOEXEC when faced with a
+                // shebang-less script. Disable posix_spawn on OpenBSD.
+                false
+            } else {
+                target.has_header("spawn.h")
+            }
+        }),
         ("small_main_stack", &has_small_stack),
         ("use_prebuilt_docs", &|_| {
             env_var("FISH_USE_PREBUILT_DOCS").is_some_and(|v| v == "TRUE")

src/env_dispatch.rs

@@ -311,7 +311,7 @@ fn handle_term_change(vars: &EnvStack) {
 
 fn handle_fish_use_posix_spawn_change(vars: &EnvStack) {
     // Note that if the variable is missing or empty we default to true (if allowed).
-    if !allow_use_posix_spawn() {
+    if !cfg!(have_posix_spawn) {
         USE_POSIX_SPAWN.store(false, Ordering::Relaxed);
     } else if let Some(var) = vars.get(L!("fish_use_posix_spawn")) {
         let use_posix_spawn =
@@ -538,20 +538,3 @@ fn init_locale(vars: &EnvStack) {
 pub fn use_posix_spawn() -> bool {
     USE_POSIX_SPAWN.load(Ordering::Relaxed)
 }
-
-/// Whether or not we are running on an OS where we allow ourselves to use `posix_spawn()`.
-const fn allow_use_posix_spawn() -> bool {
-    // OpenBSD's posix_spawn returns status 127 instead of erroring with ENOEXEC when faced with a
-    // shebang-less script. Disable posix_spawn on OpenBSD.
-    if cfg!(target_os = "openbsd") {
-        false
-    } else if cfg!(not(target_os = "linux")) {
-        true
-    } else {
-        // The C++ code used __GLIBC_PREREQ(2, 24) && !defined(__UCLIBC__) to determine if we'll use
-        // posix_spawn() by default on Linux. Surprise! We don't have to worry about porting that
-        // logic here because the libc crate only supports 2.26+ atm.
-        // See https://github.com/rust-lang/libc/issues/1412
-        true
-    }
-}