diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 37d5271b..f992215f 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -331,6 +331,26 @@ jobs:
           file "$RUNNER_TEMP/cross/openkal-same-source.exe" | grep -q 'PE32+ executable'
           file "$RUNNER_TEMP/cross/openkal-same-source"     | grep -q 'Mach-O 64-bit arm64'
 
+      # ⭐ AND A TRANSLATION UNIT THAT INCLUDES A STANDARD HEADER, FOR THE SAME TWO.
+      #
+      # `same-source` imports std, and the std module is compiled with this
+      # package's own flags, so it cannot see what a consumer's translation unit
+      # sees. `examples/cxx` includes <filesystem> the way every module wrapper
+      # of a header library does, and that path reaches libc++'s musl locale
+      # support, which calls strtof_l and vasprintf. musl declares those only
+      # under _GNU_SOURCE, and Clang predefines _GNU_SOURCE for C++ on Linux
+      # targets alone -- so this example ran on Linux and did not compile for
+      # either of these targets until __config_site stated it.
+      - name: A standard header, built here for Windows and for macOS
+        if: matrix.toolchain == 'llvm@22.1.8'
+        run: |
+          set -euo pipefail
+          cd examples/cxx
+          for t in x86_64-windows-gnu aarch64-macos; do
+            rm -rf target
+            mcpp build --target "$t"
+          done
+
       - uses: actions/upload-artifact@v4
         if: matrix.toolchain == 'llvm@22.1.8'
         with:
diff --git a/README.md b/README.md
index 7d852ab2..07112b0b 100644
--- a/README.md
+++ b/README.md
@@ -39,8 +39,18 @@ position in it is a **claim about the environment beneath**:
 #define _LIBCPP_HAS_RANDOM_DEVICE 1   /* openkal.random reaches a source   */
 #define _LIBCPP_HAS_FILESYSTEM    1   /* openkal.fs, and its sources built */
 #define _LIBCPP_HAS_TERMINAL      1   /* isatty answers, rather than lying */
+#define _GNU_SOURCE               1   /* musl's GNU interfaces, on every target */
 ```
 
+The last line is not a libc++ switch, and it is here for the same reason as the
+others. This configuration's locale support calls `strtof_l`, `strtod_l`,
+`strtold_l` and `vasprintf`, which musl declares only under `_GNU_SOURCE`.
+Clang predefines `_GNU_SOURCE` for C++ on Linux targets and on no other, so a
+translation unit that includes a standard header — which is what every module
+wrapper of a header library does — built for Linux and failed for macOS and
+Windows with `no member named 'strtof_l' in the global namespace`. Stating it
+in the configuration gives every target what the Linux target already had.
+
 The first was measured rather than assumed. With it at `0` — the value a
 toolchain configured for glibc ships — a translation unit that includes
 `<vector>` fails with twenty errors, of which the first names the cause:
diff --git a/llvm-generated/generic/__config_site b/llvm-generated/generic/__config_site
index 7ee73e58..0589797c 100644
--- a/llvm-generated/generic/__config_site
+++ b/llvm-generated/generic/__config_site
@@ -9,6 +9,18 @@
 #ifndef _LIBCPP___CONFIG_SITE
 #define _LIBCPP___CONFIG_SITE
 
+// openkal: the C library beneath is musl, and this configuration's locale
+// support (__locale_dir/support/linux.h) calls strtof_l, strtod_l, strtold_l and
+// vasprintf, which musl declares only under _GNU_SOURCE. Clang predefines
+// _GNU_SOURCE for C++ on Linux targets and on no other, so a translation unit
+// that includes a standard header -- as every module wrapper of a header
+// library does -- compiled for the Linux target and failed for the macOS and
+// Windows targets. Stating it here gives every target the environment the
+// Linux target already had; this header is included before any C header.
+#ifndef _GNU_SOURCE
+#  define _GNU_SOURCE 1
+#endif
+
 #define _LIBCPP_ABI_VERSION 1
 #define _LIBCPP_ABI_NAMESPACE __1
 #define _LIBCPP_ABI_FORCE_ITANIUM 0
diff --git a/mcpp.toml b/mcpp.toml
index b3f3b6a1..0f1b172f 100644
--- a/mcpp.toml
+++ b/mcpp.toml
@@ -1,7 +1,7 @@
 [package]
 namespace   = "mcpplibs"
 name        = "openkal-llvm-runtime"
-version     = "0.9.1"
+version     = "0.9.2"
 description = "LLVM's C++ runtime libraries — libc++, libc++abi and libunwind — configured for openkal-musl rather than for a host C library."
 license     = "Apache-2.0"
 authors     = ["mcpplibs"]