[libpng17] Port recent changes from libpng-1.6.2.

configure.ac

@@ -334,11 +334,6 @@ AC_ARG_WITH(libpng-prefix,
     fi])
 AM_CONDITIONAL([DO_PNG_PREFIX], [test "${with_libpng_prefix:-no}" != "no"])
 
-# Because GCC by default assembles code with an executable stack, even though it
-# compiles C code with a non-executable stack, it is necessary to do a fixup
-# here (this may by GCC specific)
-AC_SUBST([AM_CCASFLAGS], [-Wa,--noexecstack])
-
 AC_ARG_ENABLE([arm-neon],
    AS_HELP_STRING([[[--enable-arm-neon]]],
       [Enable ARM NEON optimizations: =off, check, api, on:]
@@ -347,9 +342,12 @@ AC_ARG_ENABLE([arm-neon],
       [a call to png_set_option; on: turn on unconditionally.  The]
       [default is 'on' if __ARM_NEON__ is defined, otherwise 'off'.]),
    [case "$enableval" in
-      off)
+      no|off)
+         # disable the default enabling on __ARM_NEON__ systems:
          AC_DEFINE([PNG_NO_ARM_NEON], [],
-                   [Disable ARM Neon optimizations]);;
+                   [Disable ARM Neon optimizations])
+         # Prevent use of the assembler files below:
+         enable_arm_neon=no;;
       check)
          AC_DEFINE([PNG_ARM_NEON_CHECK_SUPPORTED], [],
                    [Check for ARM Neon support at run-time]);;