build: relocate libidn2 strchrnul prototype into config.h

Move the macOS strchrnul declaration from a side-channel header injected
via -include into vendor/libidn2/config.h itself, gated on __APPLE__.
config.h is the canonical place where libidn2 documents its
HAVE_STRCHRNUL detection — co-locating the platform fallback prototype
there mirrors gnulib's own approach (which declares missing symbols in
its substituted <string.h>) and removes the per-platform build-flag
asymmetry plus the dedicated include path.

The Darwin-only strchrnul.c implementation is unchanged; build.zig now
just adds it on Darwin without touching the lib/ flags array.

Verified that on both `-target x86_64-macos.14.0` (no libc strchrnul)
and `-target x86_64-macos.15.4` (libc has it) a TU mimicking lookup.c
(`#include "config.h"` then call strchrnul) compiles cleanly with no
implicit-declaration error and no redeclaration warning.

build.zig

@@ -551,26 +551,23 @@ fn buildLibidn2(
         // libc itself; on musl it would also need a separate -liconv.
         mod.linkSystemLibrary("iconv", .{});
 
-        // libidn2's lib/lookup.c calls strchrnul() unconditionally — a glibc
-        // extension absent from macOS libc and not declared by <string.h>.
-        // Upstream relies on gnulib substituting <string.h> + linking
-        // gl/strchrnul.c; we don't wire up that overlay, so ship a small
-        // Darwin-only shim and inject its prototype via -include.
-        mod.addIncludePath(b.path("vendor/libidn2/darwin"));
+        // libidn2's lib/lookup.c calls strchrnul(), a glibc extension that
+        // macOS libc lacked before 15.4 and that lookup.c never gets a
+        // declaration for (it doesn't include <string.h>). Upstream solves
+        // this through gnulib's substituted <string.h> + gl/strchrnul.c;
+        // we don't wire up that overlay. Provide our own implementation —
+        // the matching prototype is declared in vendor/libidn2/config.h
+        // under #ifdef __APPLE__, so every libidn2 TU sees it via the
+        // existing #include <config.h>.
         lib.addCSourceFile(.{
             .file = b.path("vendor/libidn2/darwin/strchrnul.c"),
             .flags = &.{},
         });
     }
 
-    const lib_flags: []const []const u8 = if (is_darwin)
-        &.{ "-DHAVE_CONFIG_H", "-DIDN2_STATIC", "-include", "strchrnul.h" }
-    else
-        &.{ "-DHAVE_CONFIG_H", "-DIDN2_STATIC" };
-
     lib.addCSourceFiles(.{
         .root = dep.path("lib"),
-        .flags = lib_flags,
+        .flags = &.{ "-DHAVE_CONFIG_H", "-DIDN2_STATIC" },
         .files = &.{
             "bidi.c",     "context.c",  "data.c",   "decode.c",
             "error.c",    "free.c",     "idna.c",   "lookup.c",

vendor/libidn2/config.h

@@ -429,6 +429,17 @@
 /* Define to 1 if you have the `strchrnul' function. */
 #define HAVE_STRCHRNUL 1
 
+/* lib/lookup.c calls strchrnul() but never #include <string.h>, so it relies
+   on a declaration being visible by the time it processes config.h. Upstream
+   gets it from gnulib's substituted <string.h>; we don't wire up that overlay
+   (see build.zig::buildLibidn2). On macOS the symbol is also missing from
+   libc before 15.4 — build.zig adds vendor/libidn2/darwin/strchrnul.c on
+   Darwin to satisfy the link. Declare the prototype here so every libidn2
+   TU that includes config.h sees a real declaration before preprocessing. */
+#ifdef __APPLE__
+char *strchrnul(const char *s, int c_in);
+#endif
+
 /* Define if you have `strerror_r'. */
 #define HAVE_STRERROR_R 1
 

vendor/libidn2/darwin/strchrnul.c

@@ -1,15 +1,14 @@
-/* macOS libc lacks strchrnul (a glibc extension). libidn2's lib/lookup.c
-   calls it directly to walk DNS label boundaries. Upstream's portable build
-   relies on gnulib substituting <string.h> with a declaration and linking
-   gl/strchrnul.c, but this build does not wire up that overlay. We provide
-   a small Darwin-only shim with the same semantics.
+/* Darwin-only strchrnul shim for libidn2.
 
-   gnulib's strchrnul.c falls through to rawmemchr() when the search byte is
-   NUL — also a glibc extension. libidn2 only ever searches for '.', so a
-   straightforward byte scan is sufficient and avoids pulling in a second
-   shim. */
+   strchrnul is a glibc extension. On macOS it is missing from libc before
+   15.4 and is never made visible to libidn2's lib/lookup.c (which calls it
+   without including <string.h>). The matching prototype is declared in
+   vendor/libidn2/config.h under #ifdef __APPLE__, so callers compile;
+   this file provides the symbol so they link.
 
-#include "strchrnul.h"
+   gnulib's strchrnul.c falls through to rawmemchr() when the search byte
+   is NUL — also a glibc extension. libidn2 only ever searches for '.', so
+   a straight byte scan is enough and avoids dragging in a second shim. */
 
 char *strchrnul(const char *s, int c_in) {
     const unsigned char c = (unsigned char) c_in;

vendor/libidn2/darwin/strchrnul.h

@@ -1,8 +0,0 @@
-/* Prototype for the macOS strchrnul shim. See strchrnul.c. */
-
-#ifndef LPD_LIBIDN2_DARWIN_STRCHRNUL_H
-#define LPD_LIBIDN2_DARWIN_STRCHRNUL_H
-
-char *strchrnul(const char *s, int c_in);
-
-#endif