[libpng16] Removed PNG_FILTER_OPTIMIZATIONS and PNG_ARM_NEON_SUPPORTED from

pnglibconf.h, allowing more of the decisions to be made internally
(pngpriv.h) during the compile.  Without this, symbol prefixing is broken
under certain circumstances on ARM platforms.  Now only the API parts of
the optimizations ('check' vs 'api') are exposed in the public header files
except that the new setting PNG_ARM_NEON_OPT documents how libpng makes the
decision about whether or not to use the optimizations.

arm/arm_init.c

@@ -16,7 +16,7 @@
 
 #include "../pngpriv.h"
 
-#ifdef PNG_ARM_NEON_SUPPORTED
+#if PNG_ARM_NEON_OPT > 0
 #ifdef PNG_ARM_NEON_CHECK_SUPPORTED /* Do run-time checks */
 #include <signal.h> /* for sig_atomic_t */
 
@@ -216,4 +216,4 @@ png_init_filter_functions_neon(png_structp pp, unsigned int bpp)
           png_read_filter_row_paeth4_neon;
    }
 }
-#endif /* PNG_ARM_NEON_SUPPORTED */
+#endif /* PNG_ARM_NEON_OPT > 0 */