Fix strftime build with GCC 8

[Joseph Myers]

Building with current GCC mainline fails with:

strftime_l.c: In function '__strftime_internal':
strftime_l.c:719:4: error: macro expands to multiple statements [-Werror=multistatement-macros]
    digits = d > width ? d : width;          \
    ^
strftime_l.c:1260:6: note: in expansion of macro 'DO_NUMBER'
      DO_NUMBER (1, tp->tm_year + TM_YEAR_BASE);
      ^~~~~~~~~
strftime_l.c:1259:4: note: some parts of macro expansion are not guarded by this 'else' clause
    else
    ^~~~

In fact this particular instance is harmless; the code looks like:

          if (modifier == L_('O'))
            goto bad_format;
          else
            DO_NUMBER (1, tp->tm_year + TM_YEAR_BASE);

and because of the goto, it doesn't matter that part of the expansion
isn't under the "else" conditional.  But it's also clearly bad style
to rely on that.  This patch changes DO_NUMBER and DO_NUMBER_SPACEPAD
to use do { } while (0) to avoid such problems.

Tested (full testsuite) for x86_64 (GCC 6); testing with
build-many-glibcs.py with GCC mainline, in conjunction with my libgcc
patch <https://gcc.gnu.org/ml/gcc-patches/2017-06/msg02032.html>.  I
intend to commit if the build-many-glibcs.py testing passes.

2017-06-27  Joseph Myers  <joseph@codesourcery.com>

	* time/strftime_l.c (DO_NUMBER): Define using do { } while (0).
	(DO_NUMBER_SPACEPAD): Likewise.

diff --git a/time/strftime_l.c b/time/strftime_l.c
index 439b971..b5ba9ca 100644
--- a/time/strftime_l.c
+++ b/time/strftime_l.c
@@ -715,12 +715,22 @@ __strftime_internal (CHAR_T *s, size_t maxsize, const CHAR_T *format,
       format_char = *f;
       switch (format_char)
 	{
-#define DO_NUMBER(d, v) \
-	  digits = d > width ? d : width;				      \
-	  number_value = v; goto do_number
-#define DO_NUMBER_SPACEPAD(d, v) \
-	  digits = d > width ? d : width;				      \
-	  number_value = v; goto do_number_spacepad
+#define DO_NUMBER(d, v)				\
+	  do					\
+	    {					\
+	      digits = d > width ? d : width;	\
+	      number_value = v;			\
+	      goto do_number;			\
+	    }					\
+	  while (0)
+#define DO_NUMBER_SPACEPAD(d, v)		\
+	  do					\
+	    {					\
+	      digits = d > width ? d : width;	\
+	      number_value = v;			\
+	      goto do_number_spacepad;		\
+	    }					\
+	  while (0)
 
 	case L_('%'):
 	  if (modifier != 0)

-- 
Joseph S. Myers
joseph@codesourcery.com