[libpng16] Reformatted the new code in configure.ac

ANNOUNCE

-Libpng 1.6.22beta04 - March 9, 2016
+Libpng 1.6.22beta04 - March 15, 2016
 
 This is not intended to be a public release.  It will be replaced
 within a few weeks by a public version or by another test version.
@@ -77,7 +77,8 @@ Version 1.6.22beta03 [March 9, 2016]
     meaningful fashion (John Bowler).
   Fixed some misleading indentation (Krishnaraj Bhat).
 
-Version 1.6.22beta04 [March 9, 2016]
+Version 1.6.22beta04 [March 15, 2016]
+  Force GCC compilation to C89 if needed (Dagobert Michelsen).
 
 Send comments/corrections/commendations to png-mng-implement at lists.sf.net
 (subscription required; visit

CHANGES

@@ -5537,7 +5537,8 @@ Version 1.6.22beta03 [March 9, 2016]
     meaningful fashion (John Bowler).
   Fixed some misleading indentation (Krishnaraj Bhat).
 
-Version 1.6.22beta04 [March 9, 2016]
+Version 1.6.22beta04 [March 15, 2016]
+  Force GCC compilation to C89 if needed (Dagobert Michelsen).
 
 Send comments/corrections/commendations to png-mng-implement at lists.sf.net
 (subscription required; visit

configure.ac

@@ -117,15 +117,20 @@ AC_ARG_ENABLE(werror,
 # For GCC 5 the default mode for C is -std=gnu11 instead of -std=gnu89
 # In pngpriv.h we request just the POSIX 1003.1 and C89 APIs by defining _POSIX_SOURCE to 1
 # This is incompatible with the new default mode, so we test for that and force the 
+# "-std=c89" compiler option:
 AC_MSG_CHECKING([if we need to force back C standard to C89])
-AC_COMPILE_IFELSE([AC_LANG_PROGRAM([#define _POSIX_SOURCE 1
-#include <stdio.h>
-])],AC_MSG_RESULT(no),[
-if test "x$GCC" != "xyes"; then
-  AC_MSG_ERROR([Forcing back to C89 is required but the flags are unknown for other compilers than GCC])
-fi
-AC_MSG_RESULT(yes)
-CFLAGS="$CFLAGS -std=c89"
+AC_COMPILE_IFELSE(
+   [AC_LANG_PROGRAM([
+      [#define _POSIX_SOURCE 1]
+      [#include <stdio.h>]
+   ])],
+   AC_MSG_RESULT(no),[
+      if test "x$GCC" != "xyes"; then
+         AC_MSG_ERROR(
+            [Forcing back to C89 is required but the flags are only known for GCC])
+      fi
+   AC_MSG_RESULT(yes)
+   CFLAGS="$CFLAGS -std=c89"
 ])
 
 # Checks for header files.