Changing the png lib to always be built in Release mode.

Always build in release mode since the symlinks on Linux don't
work right for debug mode (libpng.{so,a} points to libpng14.{so,a}
instead of libpng14d.{so,a}).

Projects/png.cmake

@@ -5,4 +5,8 @@ add_external_project(png
     -DPNG_TESTS:BOOL=OFF
     # VTK uses API that gets hidden when PNG_NO_STDIO is TRUE (default).
     -DPNG_NO_STDIO:BOOL=OFF
+    # Always build in release mode since the symlinks on Linux don't
+    # work right for debug mode (libpng.{so,a} points to libpng14.{so,a}
+    # instead of libpng14d.{so,a}).
+    -DCMAKE_BUILD_TYPE:STRING=Release
   )