Update from Gnulib

This incorporates:
2020-01-18 Rename ~~gnulib.m4 to zzgnulib.m4
2020-01-18 Fix "m4_require: circular dependency of AC_LANG_COMPILER(C)"
2020-01-18 Ensure Automake does not drop ~~gnulib.m4
2020-01-18 Fix major regression from 2020-01-10
* m4/00gnulib.m4, m4/gnulib-common.m4: Copy from Gnulib.
* m4/gnulib-comp.m4: Regenerate.
* m4/zzgnulib.m4: New file, from Gnulib.

m4/00gnulib.m4

@@ -1,4 +1,4 @@
-# 00gnulib.m4 serial 5
+# 00gnulib.m4 serial 7
 dnl Copyright (C) 2009-2020 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -48,24 +48,34 @@ m4_version_prereq([2.63.263], [],
 # mode.  As of clang 9.0, these "known" functions are identified through
 # LIBBUILTIN invocations in the LLVM source file
 # llvm/tools/clang/include/clang/Basic/Builtins.def.
+# It's not possible to AC_REQUIRE the extra tests from AC_CHECK_DECL,
+# because AC_CHECK_DECL, like other Autoconf built-ins, is not supposed
+# to AC_REQUIRE anything: some configure.ac files have their first
+# AC_CHECK_DECL executed conditionally.  Therefore append the extra tests
+# to AC_PROG_CC.
 AC_DEFUN([gl_COMPILER_CLANG],
 [
-  AC_REQUIRE([AC_PROG_CC])
+dnl AC_REQUIRE([AC_PROG_CC])
   AC_CACHE_CHECK([whether the compiler is clang],
     [gl_cv_compiler_clang],
-    [AC_EGREP_CPP([barfbarf],[
-#ifdef __clang__
-barfbarf
-#endif
-       ],
-       [gl_cv_compiler_clang=yes],
-       [gl_cv_compiler_clang=no])
+    [dnl Use _AC_COMPILE_IFELSE instead of AC_EGREP_CPP, to avoid error
+     dnl "circular dependency of AC_LANG_COMPILER(C)" if AC_PROG_CC has
+     dnl not yet been invoked.
+     _AC_COMPILE_IFELSE(
+        [AC_LANG_PROGRAM([[
+           #ifdef __clang__
+           barfbarf
+           #endif
+           ]],[[]])
+        ],
+        [gl_cv_compiler_clang=no],
+        [gl_cv_compiler_clang=yes])
     ])
 ])
-AC_DEFUN_ONCE([gl_COMPILER_PREPARE_CHECK_DECL],
+AC_DEFUN([gl_COMPILER_PREPARE_CHECK_DECL],
 [
-  AC_REQUIRE([AC_PROG_CC])
-  AC_REQUIRE([gl_COMPILER_CLANG])
+dnl AC_REQUIRE([AC_PROG_CC])
+dnl AC_REQUIRE([gl_COMPILER_CLANG])
   AC_CACHE_CHECK([for compiler option needed when checking for declarations],
     [gl_cv_compiler_check_decl_option],
     [if test $gl_cv_compiler_clang = yes; then
@@ -73,7 +83,9 @@ AC_DEFUN_ONCE([gl_COMPILER_PREPARE_CHECK_DECL],
        dnl '-Werror=implicit-function-declaration'.
        save_ac_compile="$ac_compile"
        ac_compile="$ac_compile -Werror=implicit-function-declaration"
-       AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[]],[[]])],
+       dnl Use _AC_COMPILE_IFELSE instead of AC_COMPILE_IFELSE, to avoid a
+       dnl warning "AC_COMPILE_IFELSE was called before AC_USE_SYSTEM_EXTENSIONS".
+       _AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[]],[[]])],
          [gl_cv_compiler_check_decl_option='-Werror=implicit-function-declaration'],
          [gl_cv_compiler_check_decl_option=none])
        ac_compile="$save_ac_compile"
@@ -88,18 +100,15 @@ AC_DEFUN_ONCE([gl_COMPILER_PREPARE_CHECK_DECL],
   fi
 ])
 dnl Redefine _AC_CHECK_DECL_BODY so that it references ac_compile_for_check_decl
-dnl instead of ac_compile.
+dnl instead of ac_compile.  If, for whatever reason, the override of AC_PROG_CC
+dnl in zzgnulib.m4 is inactive, use the original ac_compile.
 m4_define([_AC_CHECK_DECL_BODY],
 [  ac_save_ac_compile="$ac_compile"
-  ac_compile="$ac_compile_for_check_decl"]
+  if test -n "$ac_compile_for_check_decl"; then
+    ac_compile="$ac_compile_for_check_decl"
+  fi]
 m4_defn([_AC_CHECK_DECL_BODY])[  ac_compile="$ac_save_ac_compile"
 ])
-  ])
-dnl Redefine AC_CHECK_DECL so that it starts with an invocation of
-dnl gl_COMPILER_PREPARE_CHECK_DECL.
-m4_define([AC_CHECK_DECL],
-  [gl_COMPILER_PREPARE_CHECK_DECL dnl
-]m4_defn([AC_CHECK_DECL]))
 
 # gl_00GNULIB
 # -----------

m4/gnulib-common.m4

@@ -1,4 +1,4 @@
-# gnulib-common.m4 serial 46
+# gnulib-common.m4 serial 47
 dnl Copyright (C) 2007-2020 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -12,6 +12,7 @@ AC_DEFUN([gl_COMMON], [
   dnl Use AC_REQUIRE here, so that the code is expanded once only.
   AC_REQUIRE([gl_00GNULIB])
   AC_REQUIRE([gl_COMMON_BODY])
+  AC_REQUIRE([gl_ZZGNULIB])
 ])
 AC_DEFUN([gl_COMMON_BODY], [
   AH_VERBATIM([_Noreturn],

m4/gnulib-comp.m4

@@ -1159,4 +1159,5 @@ AC_DEFUN([gl_FILE_LIST], [
   m4/warnings.m4
   m4/wchar_t.m4
   m4/wint_t.m4
+  m4/zzgnulib.m4
 ])

m4/zzgnulib.m4

@@ -0,0 +1,23 @@
+# zzgnulib.m4 serial 1
+dnl Copyright (C) 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.
+
+dnl This file must be named something that sorts after all other
+dnl package- or gnulib-provided .m4 files - at least for those packages
+dnl that redefine AC_PROG_CC.
+
+dnl Redefine AC_PROG_CC so that it ends with invocations of gl_COMPILER_CLANG
+dnl and gl_COMPILER_PREPARE_CHECK_DECL.
+m4_define([AC_PROG_CC],
+  m4_defn([AC_PROG_CC])[
+gl_COMPILER_CLANG
+gl_COMPILER_PREPARE_CHECK_DECL
+])
+
+# gl_ZZGNULIB
+# -----------
+# Witness macro that this file has been included.  Needed to force
+# Automake to include this file after all other gnulib .m4 files.
+AC_DEFUN([gl_ZZGNULIB])