3 files changed, 32 insertions, 24 deletions
diff --git a/Build/source/libs/libpng/ChangeLog b/Build/source/libs/libpng/ChangeLog
index 37a47ac3093..d7ce3a6e24b 100644
--- a/Build/source/libs/libpng/ChangeLog
+++ b/Build/source/libs/libpng/ChangeLog
@@ -1,3 +1,8 @@
+2018-01-14  Karl Berry  <karl@freefriends.org>
+
+	* configure.ac (-std=c89): reinstate check, now copying configure
+	block from upstream; reautoconf.
+
 2017-12-02  Karl Berry  <karl@freefriends.org>
 
 	Import ARM support from upstream to TL configure.
diff --git a/Build/source/libs/libpng/configure b/Build/source/libs/libpng/configure
index 3a48f580932..ee9bc6a9747 100755
--- a/Build/source/libs/libpng/configure
+++ b/Build/source/libs/libpng/configure
@@ -4025,6 +4025,9 @@ WARNING_CFLAGS=$kpse_cv_warning_cflags
 
 
 
+ac_config_headers="$ac_config_headers config.h"
+
+
 ac_ext=c
 ac_cpp='$CPP $CPPFLAGS'
 ac_compile='$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&5'
@@ -5039,19 +5042,17 @@ esac
 
 
 
-ac_config_headers="$ac_config_headers config.h"
-
-
-if false; then
 # 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:
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking if we need to force back C standard to C89" >&5
 $as_echo_n "checking if we need to force back C standard to C89... " >&6; }
 cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
-#define _POSIX_SOURCE 1
-#include <stdio.h>
+
+      #define _POSIX_SOURCE 1
+      #include <stdio.h>
 
 int
 main ()
@@ -5066,16 +5067,15 @@ if ac_fn_c_try_compile "$LINENO"; then :
 $as_echo "no" >&6; }
 else
 
-if test "x$GCC" != "xyes"; then
-  as_fn_error $? "Forcing back to C89 is required but the flags are unknown for other compilers than GCC" "$LINENO" 5
-fi
-{ $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
+      if test "x$GCC" != "xyes"; then
+         as_fn_error $? "Forcing back to C89 is required but the flags are only known for GCC" "$LINENO" 5
+      fi
+   { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
 $as_echo "yes" >&6; }
-CFLAGS="$CFLAGS -std=c89"
+   CFLAGS="$CFLAGS -std=c89"
 
 fi
 rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
-fi
 
 # Checks for header files.
 ac_ext=c
diff --git a/Build/source/libs/libpng/configure.ac b/Build/source/libs/libpng/configure.ac
index 05ae777d344..bfb9836dd12 100644
--- a/Build/source/libs/libpng/configure.ac
+++ b/Build/source/libs/libpng/configure.ac
@@ -17,6 +17,8 @@ AC_CONFIG_MACRO_DIR([../../m4])
 
 KPSE_BASIC([libpng])
 
+AC_CONFIG_HEADERS([config.h])
+
 AC_PROG_CC
 AM_PROG_AS
 AC_PROG_RANLIB
@@ -24,23 +26,24 @@ AC_PROG_LN_S
 
 KPSE_COMPILER_VISIBILITY
 
-AC_CONFIG_HEADERS([config.h])
-
-if false; then
 # 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"
 ])
-fi
 
 # Checks for header files.
 AC_HEADER_STDC