Follow the steps below to build pngquant from source on MinGW (tested Win 7 64-bit).

You can skip first two steps if you have libpng installed on your system
(although you may need to change paths in the Makefile)

1. Download the zlib source code, developer files and binaries ZIPs from:
   http://gnuwin32.sourceforge.net/packages/zlib.htm

   Unzip the source ZIP and locate the sub-folder containing various source files 
   incl. zlib.h - copy this folder into the parent folder of the 'pngquant' 
   folder and rename it to 'zlib'.

   Unzip the dev files ZIP and copy the "lib" sub-folder to the "zlib" folder.

2. Download the libpng source code, developer files and binaries ZIPs from:
   http://gnuwin32.sourceforge.net/packages/libpng.htm

   Unzip the source ZIP and locate the sub-folder containing various source files 
   incl. png.h - copy this folder into the parent folder of the 'pngquant' folder 
   and rename it to 'libpng'.

   Unzip the dev files ZIP and copy the "lib" sub-folder to the "libpng" folder.

3. Run the MinGW Shell (MSYS) and navigate to the pngquant folder; e.g.:

   cd "c:\pngquant-src\pngquant\"

4. Type "make" to begin compilation. If is successful (should be!), a "bin" 
   sub-folder will be created, containing "pngquanti.exe". If it fails, Google 
   the error messages (but can ignore warnings - "missing braces" etc).

5. From the zlib/libpng binaries ZIPs, copy the "bin/*.dll" files - probably
   just need "libpng3.dll" and "zlib1.dll", but names may vary slightly.

6. Run pnqquanti.exe - should work fine. If a missing DLL message is shown, 
   check is in same folder. Congrats!