[libpng16] Use parentheses consistently in "#if defined()" tests.

libpng-manual.txt

@@ -1,6 +1,6 @@
 libpng-manual.txt - A description on how to use and modify libpng
 
- libpng version 1.6.1beta02 - February 16, 2013
+ libpng version 1.6.1beta02 - February 17, 2013
  Updated and distributed by Glenn Randers-Pehrson
  <glennrp at users.sourceforge.net>
  Copyright (c) 1998-2013 Glenn Randers-Pehrson
@@ -11,7 +11,7 @@ libpng-manual.txt - A description on how to use and modify libpng
 
  Based on:
 
- libpng versions 0.97, January 1998, through 1.6.1beta02 - February 16, 2013
+ libpng versions 0.97, January 1998, through 1.6.1beta02 - February 17, 2013
  Updated and distributed by Glenn Randers-Pehrson
  Copyright (c) 1998-2013 Glenn Randers-Pehrson
 
@@ -4953,7 +4953,7 @@ The following API are now DEPRECATED:
    png_free_default()
    png_reset_zstream()
 
-The following has been removed:
+The following have been removed:
    png_get_io_chunk_name(), which has been replaced
      with png_get_io_chunk_type().  The new
      function returns a 32-bit integer instead of
@@ -5134,7 +5134,8 @@ left parenthesis that follows it:
        y[i] = a(x) + (int)b;
 
 We prefer #ifdef and #ifndef to #if defined() and #if !defined()
-when there is only one macro being tested.
+when there is only one macro being tested.  We always use parentheses
+with "defined".
 
 We prefer to express integers that are used as bit masks in hex format,
 with an even number of lower-case hex digits (e.g., 0x00, 0xff, 0x0100).
@@ -5150,7 +5151,7 @@ Other rules can be inferred by inspecting the libpng source.
 
 XVI. Y2K Compliance in libpng
 
-February 16, 2013
+February 17, 2013
 
 Since the PNG Development group is an ad-hoc body, we can't make
 an official declaration.