Use system libpng and libmng by default

(cherry picked from commit b129c37241)

configure

@@ -111,11 +111,11 @@ CFG_XINERAMA=auto
 CFG_BIG_CODECS=yes
 CFG_ZLIB=yes
 CFG_PNG=yes
-CFG_LIBPNG=qt
+CFG_LIBPNG=system
 CFG_JPEG=auto
 CFG_LIBJPEG=system
 CFG_MNG=auto
-CFG_LIBMNG=qt
+CFG_LIBMNG=system
 CFG_XCURSOR=auto
 CFG_XRANDR=auto
 CFG_XRENDER=auto
@@ -1998,12 +1998,12 @@ Image Formats:
 
 Third Party Image Loading Libraries:
 
- *  -qt-libpng ......... Use the libpng bundled with Qt.
+    -qt-libpng ......... Use the libpng bundled with Qt.
-    -system-libpng ..... Use libpng from the operating system.
+ *  -system-libpng ..... Use libpng from the operating system.
 			 See http://www.libpng.org/pub/png
 
- *  -qt-libmng ......... Use the libmng bundled with Qt.
+    -qt-libmng ......... Use the libmng bundled with Qt.
-    -system-libmng ..... Use libmng from the operating system.
+ *  -system-libmng ..... Use libmng from the operating system.
 			 See http://www.libmng.com
 EOF