Fix cross-compilation test results

[Bruno Haible]

Before testing __GNU_LIBRARY__, one has to include either <features.h>
(which only exists on glibc, musl, Android, and Cygwin systems) or some
header file such as <stdlib.h> or <string.h>.


2021-07-17  Bruno Haible  <bruno@clisp.org>

        Fix cross-compilation test results.
        * m4/memmem.m4 (gl_FUNC_MEMMEM_SIMPLE): When cross-compiling, include
        <string.h> before testing __GNU_LIBRARY__.
        * m4/strstr.m4 (gl_FUNC_STRSTR_SIMPLE): Likewise.
        * m4/strcasestr.m4 (gl_FUNC_STRCASESTR_SIMPLE): Likewise.

diff --git a/m4/memmem.m4 b/m4/memmem.m4
index f4a167b..6dac766 100644
--- a/m4/memmem.m4
+++ b/m4/memmem.m4
@@ -1,4 +1,4 @@
-# memmem.m4 serial 28
+# memmem.m4 serial 29
 dnl Copyright (C) 2002-2004, 2007-2021 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -50,6 +50,7 @@ AC_DEFUN([gl_FUNC_MEMMEM_SIMPLE],
          dnl Assume that it works on all other platforms (even if not linear).
          AC_EGREP_CPP([Lucky user],
            [
+#include <string.h> /* for __GNU_LIBRARY__ */
 #ifdef __GNU_LIBRARY__
  #include <features.h>
  #if ((__GLIBC__ == 2 && ((__GLIBC_MINOR > 0 && __GLIBC_MINOR__ < 9) \
diff --git a/m4/strcasestr.m4 b/m4/strcasestr.m4
index e9dc3e2..6939144 100644
--- a/m4/strcasestr.m4
+++ b/m4/strcasestr.m4
@@ -1,4 +1,4 @@
-# strcasestr.m4 serial 27
+# strcasestr.m4 serial 28
 dnl Copyright (C) 2005, 2007-2021 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -26,7 +26,7 @@ AC_DEFUN([gl_FUNC_STRCASESTR_SIMPLE],
         [gl_cv_func_strcasestr_works_always],
         [AC_RUN_IFELSE(
            [AC_LANG_PROGRAM([[
-#include <string.h> /* for strcasestr */
+#include <string.h> /* for __GNU_LIBRARY__, strcasestr */
 #ifdef __GNU_LIBRARY__
  #include <features.h>
  #if __GLIBC__ == 2 && __GLIBC_MINOR__ == 28
@@ -48,6 +48,7 @@ AC_DEFUN([gl_FUNC_STRCASESTR_SIMPLE],
             dnl linear.
             AC_EGREP_CPP([Lucky user],
               [
+#include <string.h> /* for __GNU_LIBRARY__ */
 #ifdef __GNU_LIBRARY__
  #include <features.h>
  #if ((__GLIBC__ == 2 && __GLIBC_MINOR__ > 12) || (__GLIBC__ > 2)) \
diff --git a/m4/strstr.m4 b/m4/strstr.m4
index 9604fe2..a32e9c1 100644
--- a/m4/strstr.m4
+++ b/m4/strstr.m4
@@ -1,4 +1,4 @@
-# strstr.m4 serial 23
+# strstr.m4 serial 24
 dnl Copyright (C) 2008-2021 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -18,7 +18,7 @@ AC_DEFUN([gl_FUNC_STRSTR_SIMPLE],
       [gl_cv_func_strstr_works_always],
       [AC_RUN_IFELSE(
          [AC_LANG_PROGRAM([[
-#include <string.h> /* for strstr */
+#include <string.h> /* for __GNU_LIBRARY__, strstr */
 #ifdef __GNU_LIBRARY__
  #include <features.h>
  #if __GLIBC__ == 2 && __GLIBC_MINOR__ == 28
@@ -40,6 +40,7 @@ AC_DEFUN([gl_FUNC_STRSTR_SIMPLE],
           dnl linear.
           AC_EGREP_CPP([Lucky user],
             [
+#include <string.h> /* for __GNU_LIBRARY__ */
 #ifdef __GNU_LIBRARY__
  #include <features.h>
  #if ((__GLIBC__ == 2 && __GLIBC_MINOR__ > 12) || (__GLIBC__ > 2)) \