diff --git a/configure.ac b/configure.ac
index 012fbd3fb..ae649b4a0 100644
--- a/configure.ac
+++ b/configure.ac
@@ -346,6 +346,10 @@ dnl ############################################################################
 dnl OS specific stuff
 dnl ############################################################################
 
+AC_CHECK_DECLS([environ], , ,
+  [ #include <unistd.h>
+    #include <stdlib.h> ])
+
 case $host_os in
 *os400)
     AC_PATH_PROG([PERL], [perl], [/QOpenSys/pkgs/bin/perl])
diff --git a/lib/widget/input_complete.c b/lib/widget/input_complete.c
index cd0ccbc87..4d3fd8b4b 100644
--- a/lib/widget/input_complete.c
+++ b/lib/widget/input_complete.c
@@ -54,8 +54,7 @@
 
 /*** global variables ****************************************************************************/
 
-/* Linux declares environ in <unistd.h>, so don't repeat it here. */
-#if (!(defined(__linux__) && defined (__USE_GNU)) && !defined(__CYGWIN__))
+#if !HAVE_DECL_ENVIRON
 extern char **environ;
 #endif
 
diff --git a/m4.include/ax_check_compile_flag.m4 b/m4.include/ax_check_compile_flag.m4
index 8323a2275..54191c553 100644
--- a/m4.include/ax_check_compile_flag.m4
+++ b/m4.include/ax_check_compile_flag.m4
@@ -34,19 +34,24 @@
 #   and this notice are preserved.  This file is offered as-is, without any
 #   warranty.
 
-# Yury Zaytsev:
-# Toggle Werror since at some point clang started treating unknown -W
-# flags as warnings, succeeding with the build, yet issuing an annoying
-# warning.
-
-#serial 6
+#serial 11
 
 AC_DEFUN([AX_CHECK_COMPILE_FLAG],
 [AC_PREREQ(2.64)dnl for _AC_LANG_PREFIX and AS_VAR_IF
 AS_VAR_PUSHDEF([CACHEVAR],[ax_cv_check_[]_AC_LANG_ABBREV[]flags_$4_$1])dnl
-AC_CACHE_CHECK([whether _AC_LANG compiler accepts $1], CACHEVAR, [
+AC_CACHE_CHECK([whether the _AC_LANG compiler accepts $1], CACHEVAR, [
   ax_check_save_flags=$[]_AC_LANG_PREFIX[]FLAGS
-  _AC_LANG_PREFIX[]FLAGS="$[]_AC_LANG_PREFIX[]FLAGS $4 $1 -Werror"
+  if test x"m4_case(_AC_LANG,
+                     [C], [$GCC],
+                     [C++], [$GXX],
+                     [Fortran], [$GFC],
+                     [Fortran 77], [$G77],
+                     [Objective C], [$GOBJC],
+                     [Objective C++], [$GOBJCXX],
+                     [no])" = xyes ; then
+    add_gnu_werror="-Werror"
+  fi
+  _AC_LANG_PREFIX[]FLAGS="$[]_AC_LANG_PREFIX[]FLAGS $4 $1 $add_gnu_werror"
   AC_COMPILE_IFELSE([m4_default([$5],[AC_LANG_PROGRAM()])],
     [AS_VAR_SET(CACHEVAR,[yes])],
     [AS_VAR_SET(CACHEVAR,[no])])