update m4/extern-inline.m4 to upstream version to fix compilation on FreeBSD

In file included from util/grub-mkimage.c:54:0:
./grub-core/gnulib/argp.h:627:49: error: '__sbistype' is static but
used in inline function '_option_is_short' which is not static
[-Werror] cc1: all warnings being treated as errors gmake[2]: ***
[util/grub_mkimage-grub-mkimage.o] Error 1

Update m4/extern-inline.m4 to current upstream gnulib version that
contains fix for this (commit b9bfe78424b871f5b92e5ee9e7d21ef951a6801d).

Reported-By: Beeblebrox <zaphod@berentweb.com>

m4/extern-inline.m4

@@ -1,6 +1,6 @@
 dnl 'extern inline' a la ISO C99.
 
-dnl Copyright 2012-2013 Free Software Foundation, Inc.
+dnl Copyright 2012-2015 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.
@@ -8,42 +8,77 @@ dnl with or without modifications, as long as this notice is preserved.
 AC_DEFUN([gl_EXTERN_INLINE],
 [
   AH_VERBATIM([extern_inline],
-[/* _GL_INLINE is a portable alternative to ISO C99 plain 'inline'.
+[/* Please see the Gnulib manual for how to use these macros.
-   _GL_EXTERN_INLINE is a portable alternative to 'extern inline'.
-   _GL_INLINE_HEADER_BEGIN contains useful stuff to put
-     in an include file, before uses of _GL_INLINE.
-     It suppresses GCC's bogus "no previous prototype for 'FOO'" diagnostic,
-     when FOO is an inline function in the header; see
-     <http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54113>.
-   _GL_INLINE_HEADER_END contains useful stuff to put
-     in the same include file, after uses of _GL_INLINE.
 
    Suppress extern inline with HP-UX cc, as it appears to be broken; see
    <http://lists.gnu.org/archive/html/bug-texinfo/2013-02/msg00030.html>.
 
-   Suppress the use of extern inline on Apple's platforms,
+   Suppress extern inline with Sun C in standards-conformance mode, as it
-   as Libc-825.25 (2012-09-19) is incompatible with it; see
+   mishandles inline functions that call each other.  E.g., for 'inline void f
-   <http://lists.gnu.org/archive/html/bug-gnulib/2012-12/msg00023.html>.
+   (void) { } inline void g (void) { f (); }', c99 incorrectly complains
-   Perhaps Apple will fix this some day.  */
+   'reference to static identifier "f" in extern inline function'.
+   This bug was observed with Sun C 5.12 SunOS_i386 2011/11/16.
+
+   Suppress extern inline (with or without __attribute__ ((__gnu_inline__)))
+   on configurations that mistakenly use 'static inline' to implement
+   functions or macros in standard C headers like <ctype.h>.  For example,
+   if isdigit is mistakenly implemented via a static inline function,
+   a program containing an extern inline function that calls isdigit
+   may not work since the C standard prohibits extern inline functions
+   from calling static functions.  This bug is known to occur on:
+
+     OS X 10.8 and earlier; see:
+     http://lists.gnu.org/archive/html/bug-gnulib/2012-12/msg00023.html
+
+     DragonFly; see
+     http://muscles.dragonflybsd.org/bulk/bleeding-edge-potential/latest-per-pkg/ah-tty-0.3.12.log
+
+     FreeBSD; see:
+     http://lists.gnu.org/archive/html/bug-gnulib/2014-07/msg00104.html
+
+   OS X 10.9 has a macro __header_inline indicating the bug is fixed for C and
+   for clang but remains for g++; see <http://trac.macports.org/ticket/41033>.
+   Assume DragonFly and FreeBSD will be similar.  */
+#if (((defined __APPLE__ && defined __MACH__) \
+      || defined __DragonFly__ || defined __FreeBSD__) \
+     && (defined __header_inline \
+         ? (defined __cplusplus && defined __GNUC_STDC_INLINE__ \
+            && ! defined __clang__) \
+         : ((! defined _DONT_USE_CTYPE_INLINE_ \
+             && (defined __GNUC__ || defined __cplusplus)) \
+            || (defined _FORTIFY_SOURCE && 0 < _FORTIFY_SOURCE \
+                && defined __GNUC__ && ! defined __cplusplus))))
+# define _GL_EXTERN_INLINE_STDHEADER_BUG
+#endif
 #if ((__GNUC__ \
       ? defined __GNUC_STDC_INLINE__ && __GNUC_STDC_INLINE__ \
-      : 199901L <= __STDC_VERSION__ && !defined __HP_cc) \
+      : (199901L <= __STDC_VERSION__ \
-     && !defined __APPLE__)
+         && !defined __HP_cc \
+         && !(defined __SUNPRO_C && __STDC__))) \
+     && !defined _GL_EXTERN_INLINE_STDHEADER_BUG)
 # define _GL_INLINE inline
 # define _GL_EXTERN_INLINE extern inline
-#elif 2 < __GNUC__ + (7 <= __GNUC_MINOR__) && !defined __APPLE__
+# define _GL_EXTERN_INLINE_IN_USE
-# if __GNUC_GNU_INLINE__
+#elif (2 < __GNUC__ + (7 <= __GNUC_MINOR__) && !defined __STRICT_ANSI__ \
+       && !defined _GL_EXTERN_INLINE_STDHEADER_BUG)
+# if defined __GNUC_GNU_INLINE__ && __GNUC_GNU_INLINE__
    /* __gnu_inline__ suppresses a GCC 4.2 diagnostic.  */
 #  define _GL_INLINE extern inline __attribute__ ((__gnu_inline__))
 # else
 #  define _GL_INLINE extern inline
 # endif
 # define _GL_EXTERN_INLINE extern
+# define _GL_EXTERN_INLINE_IN_USE
 #else
 # define _GL_INLINE static _GL_UNUSED
 # define _GL_EXTERN_INLINE static _GL_UNUSED
 #endif
 
+/* In GCC, suppress bogus "no previous prototype for 'FOO'"
+   and "no previous declaration for 'FOO'" diagnostics,
+   when FOO is an inline function in the header; see
+   <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54113> and
+   <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63877>.  */
 #if 4 < __GNUC__ + (6 <= __GNUC_MINOR__)
 # if defined __GNUC_STDC_INLINE__ && __GNUC_STDC_INLINE__
 #  define _GL_INLINE_HEADER_CONST_PRAGMA