Update from Gnulib

This is minor refactoring that should not affect Emacs builds.
It incorporates:
2018-10-12 Make better use of Autoconf
* m4/environ.m4, m4/fsusage.m4, m4/manywarnings.m4, m4/socklen.m4:
Copy from Gnulib.

m4/environ.m4

@@ -1,4 +1,4 @@
-# environ.m4 serial 6
+# environ.m4 serial 7
 dnl Copyright (C) 2001-2004, 2006-2018 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -29,16 +29,14 @@ AC_DEFUN_ONCE([gl_ENVIRON],
 AC_DEFUN([gt_CHECK_VAR_DECL],
 [
   define([gt_cv_var], [gt_cv_var_]$2[_declaration])
-  AC_MSG_CHECKING([if $2 is properly declared])
-  AC_CACHE_VAL([gt_cv_var], [
-    AC_COMPILE_IFELSE(
-      [AC_LANG_PROGRAM(
-         [[$1
-           extern struct { int foo; } $2;]],
-         [[$2.foo = 1;]])],
-      [gt_cv_var=no],
-      [gt_cv_var=yes])])
-  AC_MSG_RESULT([$gt_cv_var])
+  AC_CACHE_CHECK([if $2 is properly declared], [gt_cv_var],
+    [AC_COMPILE_IFELSE(
+       [AC_LANG_PROGRAM(
+          [[$1
+            extern struct { int foo; } $2;]],
+          [[$2.foo = 1;]])],
+       [gt_cv_var=no],
+       [gt_cv_var=yes])])
   if test $gt_cv_var = yes; then
     AC_DEFINE([HAVE_]m4_translit($2, [a-z], [A-Z])[_DECL], 1,
               [Define if you have the declaration of $2.])