gnugrep: iOS stackvma stub for vendored gnulib

gnugrep bundles gnulib's lib/stackvma.c, which introspects Mach VM
regions to implement stack-overflow detection for libsigsegv. Its
platform gate `#if defined __APPLE__ && defined __MACH__` fires on
both macOS and iOS, but the macOS arm includes `<libc.h>` and
`<nlist.h>` — BSD convenience headers Xcode ships on macOS only.

Note: `<libc.h>` here is a macOS-specific grab-bag header that
re-exports misc BSD extensions, *not* the standard C library
(`<stdlib.h>`, `<string.h>`, etc., which iOS ships normally).

`--without-libsigsegv-prefix` only disables linking against external
libsigsegv; it doesn't exclude the vendored stackvma from the build.
Replace stackvma.c with a stub `sigsegv_get_vma` returning -1 ("no
info"). iOS sandboxes forbid mach task/vm introspection anyway, so
stack-overflow detection would never work there; the stub matches
reality.

Author: insipx

pkgs/tools/text/gnugrep/default.nix

@@ -33,7 +33,11 @@ stdenv.mkDerivation {
     # https://lists.gnu.org/archive/html/bug-gnulib/2025-07/msg00021.html
     # Multiple upstream commits squashed with adjustments, see header
     ./gnulib-float-h-tests-port-to-C23-PowerPC-GCC.patch
-  ];
+  ]
+  # gnulib stackvma.c's __APPLE__ arm includes <libc.h>/<nlist.h> (macOS-only
+  # BSD headers, not libc itself). Narrow via TARGET_OS_IPHONE so iOS hits the
+  # existing `#else /* Hurd, Minix, ... */` no-op stub.
+  ++ lib.optional stdenv.hostPlatform.isiOS ./patches/ios-stackvma-skip-macos-branch.patch;
 
   # Some gnulib tests fail
   # - on Musl: https://github.com/NixOS/nixpkgs/pull/228714
@@ -63,6 +67,11 @@ stdenv.mkDerivation {
   ]
   ++ lib.optional (!stdenv.hostPlatform.isWindows) runtimeShellPackage;
 
+  # External libsigsegv isn't packaged for iOS (vendored copy patched separately).
+  configureFlags = lib.optionals stdenv.hostPlatform.isiOS [
+    "--without-libsigsegv-prefix"
+  ];
+
   # cygwin: FAIL: multibyte-white-space
   # freebsd: FAIL mb-non-UTF8-performance
   # x86_64-darwin: fails 'stack-overflow' tests on Rosetta 2 emulator

pkgs/tools/text/gnugrep/patches/ios-stackvma-skip-macos-branch.patch

@@ -0,0 +1,23 @@
+--- a/lib/stackvma.c	2025-01-01 21:33:12.000000000 -0500
++++ b/lib/stackvma.c	2026-04-23 09:37:32.499102018 -0400
+@@ -19,6 +19,10 @@
+ 
+ #include <config.h>
+ 
++#if defined __APPLE__ && defined __MACH__
++# include <TargetConditionals.h>
++#endif
++
+ /* On Solaris in 32-bit mode, when gnulib module 'largefile' is in use,
+    prevent a compilation error
+      "Cannot use procfs in the large file compilation environment"
+@@ -1361,7 +1365,8 @@
+ 
+ /* ---------------------------- stackvma-mach.c ---------------------------- */
+ 
+-#elif (defined __APPLE__ && defined __MACH__) /* macOS */
++#elif (defined __APPLE__ && defined __MACH__ \
++       && (!defined TARGET_OS_IPHONE || !TARGET_OS_IPHONE)) /* macOS, not iOS */
+ 
+ #include <libc.h>
+ #include <nlist.h>