Here is what I do to build on mingw64:

1.  Untar the pari source code in the usual way.
2.  Copy the new parigen.h into <pari_root>/src/headers/.
3.  From the src directory, run ./convertLongsAllFiles.sh
    This will replace long with pari_long throughout the entire code base.
4.  Also from the src directory, run ./convertMingw64.sh
    This will make the necessary changes to build on mingw64.
    (Eventually this could be incorporated into the Makefile as part of the build.)
5.  Copy the provided pari.cfg into <pari_root>.
    You should make sure all paths and other variables are set for your particular system.
6.  From <pari_root>, run ./Configure --load pari.cfg
7.  Compare the generated Makefile with my saved version Makefile.SAVE and make any necessary changes.
    In particular, AR and RANLIB need to be changed (I am not sure if there is a way to fix pari.cfg 
    so that the AR and RANLIB are automatically set).
8.  Finally, do "make all".


Known Issues:
1. In plotport.c there is a warning about using %ld for a pari_long.  I could not see a way to
   easily fix this within the convertMingw64 script and I believe it won't be a problem in this 
   case since longlong should be overkill anyway for a "fontsize" variable.
2. My pari.cfg has gmp disabled.  I disabled this early on when it appeared that some of the gmp
   functions required long* as inputs (implying they were doing arithmetic with longs). According 
   to my notes, one such function was mpn_gcdext.  Maybe you know of a simple way to fix this.
   Is there a longlong* version of the function which could be used in it's place?