hard-locale tests: make it easy to reuse the musl test

[Bruno Haible]

Some other tests may need to conditionalize on musl libc, in the future.


2020-01-25  Bruno Haible  <address@hidden>

        hard-locale tests: Make it easy to reuse the musl test.
        * m4/musl.m4: New file, extracted from modules/hard-locale-tests.
        * modules/hard-locale-tests (Files): Add it.
        (configure.ac): Invoke gl_MUSL_LIBC.

diff --git a/m4/musl.m4 b/m4/musl.m4
new file mode 100644
index 0000000..252054c
--- /dev/null
+++ b/m4/musl.m4
@@ -0,0 +1,18 @@
+# musl.m4 serial 1
+dnl Copyright (C) 2019-2020 Free Software Foundation, Inc.
+dnl This file is free software; the Free Software Foundation
+dnl gives unlimited permission to copy and/or distribute it,
+dnl with or without modifications, as long as this notice is preserved.
+
+# Test for musl libc, despite the musl libc authors don't like it
+# <https://wiki.musl-libc.org/faq.html>
+# <https://lists.gnu.org/archive/html/bug-gnulib/2018-02/msg00079.html>.
+# From Bruno Haible.
+
+AC_DEFUN_ONCE([gl_MUSL_LIBC],
+[
+  AC_REQUIRE([AC_CANONICAL_HOST])
+  case "$host_os" in
+    *-musl*) AC_DEFINE([MUSL_LIBC], [1], [Define to 1 on musl libc.]) ;;
+  esac
+])
diff --git a/modules/hard-locale-tests b/modules/hard-locale-tests
index ac7fcb2..7ed8575 100644
--- a/modules/hard-locale-tests
+++ b/modules/hard-locale-tests
@@ -1,14 +1,12 @@
 Files:
 tests/test-hard-locale.c
 tests/locale.c
+m4/musl.m4
 
 Depends-on:
 
 configure.ac:
-AC_REQUIRE([AC_CANONICAL_HOST])
-case "$host_os" in
-  *-musl*) AC_DEFINE([MUSL_LIBC], [1], [Define to 1 on musl libc.]) ;;
-esac
+gl_MUSL_LIBC
 dnl Distinguish OpenBSD >= 6.2 from OpenBSD < 6.2.
 AC_CHECK_FUNCS_ONCE([duplocale])