Log Message:
Changes 0.7:
* Upgraded libpng to version 1.4.9.
* Upgraded zlib to version 1.2.6-optipng.
* Resolved a rarely-occurring out-of-bounds memory access error in
  Z_RLE, by upgrading zlib.
* Added the popularly-requested option -strip.
  The only suboption currently supported is "-strip all".
* Added the option -clobber.
* Added the option -backup, as an alias of -keep.
* Added the option -silent, as an alias of -quiet.
* Deprecated the option -log.
* Changed the activity display output from STDOUT to STDERR.
* Allowed the option -preserve to save high-resolution timestamps
  on Unix, if the POSIX-1.2008 API is available.  This feature was
  previously available on Windows only.
* Fixed a minor precision error in the display of file size percents.
* Fixed a memory leak that occurred when reading broken GIF images.
* Fixed various build issues.
* Resolved all remaining compatibility issues with libpng-1.5.
* Added build support for clang.
* Reorganized the source directory structure.