gconv.h: fix build with GCC 7

gconv.h is using a flex array to define the __gconv_info member in an
invalid way, causing GCC 7 to issue an error:

| In file included from ../include/gconv.h:1:0,
|                  from ../sysdeps/unix/sysv/linux/_G_config.h:32,
|                  from ../libio/libio.h:31,
|                  from ../include/libio.h:4,
|                  from ../libio/stdio.h:74,
|                  from ../include/stdio.h:5,
|                  from test-math-isinff.cc:22:
| ../iconv/gconv.h:142:50: error: flexible array member '__gconv_info::__data' not at end of 'struct _IO_codecvt'
| In file included from ../include/libio.h:4:0,
|                  from ../libio/stdio.h:74,
|                  from ../include/stdio.h:5,
|                  from test-math-isinff.cc:22:
| ../libio/libio.h:211:14: note: next member '_G_iconv_t _IO_codecvt::__cd_out' declared here
| ../libio/libio.h:187:8: note: in the definition of 'struct _IO_codecvt'
| In file included from ../include/gconv.h:1:0,
|                  from ../sysdeps/unix/sysv/linux/_G_config.h:32,
|                  from ../libio/libio.h:31,
|                  from ../include/libio.h:4,
|                  from ../libio/stdio.h:74,
|                  from ../include/stdio.h:5,
|                  from test-math-isinff.cc:22:
| ../iconv/gconv.h:142:50: error: flexible array member '__gconv_info::__data' not at end of 'struct _IO_wide_data'
| In file included from ../include/libio.h:4:0,
|                  from ../libio/stdio.h:74,
|                  from ../include/stdio.h:5,
|                  from test-math-isinff.cc:22:
| ../libio/libio.h:211:14: note: next member '_G_iconv_t _IO_codecvt::__cd_out' declared here
| ../libio/libio.h:215:8: note: in the definition of 'struct _IO_wide_data'

This is basically a revert to the code from 15 years ago. More details
are available in the GCC bug:
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78039

Changelog:
	* iconv/gconv.h (__gconv_info): Define __data element using a
	zero-length array.

2 files changed, 6 insertions, 1 deletions

diff --git a/ChangeLog b/ChangeLog
index f737fa6b71..afe2254a64 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2016-11-06  Aurelien Jarno  <aurelien@aurel32.net>
+
+	* iconv/gconv.h (__gconv_info): Define __data element using a
+	zero-length array.
+
 2016-11-04  Steve Ellcey  <sellcey@caviumnetworks.com>
 
 	* math/test-tgmath2.c: Split up test function.
diff --git a/iconv/gconv.h b/iconv/gconv.h
index 8d8ce5813b..a87028047b 100644
--- a/iconv/gconv.h
+++ b/iconv/gconv.h
@@ -139,7 +139,7 @@ typedef struct __gconv_info
 {
   size_t __nsteps;
   struct __gconv_step *__steps;
-  __extension__ struct __gconv_step_data __data __flexarr;
+  __extension__ struct __gconv_step_data __data[0];
 } *__gconv_t;
 
 /* Transliteration using the locale's data.  */