To compile it get MinGW, which contains the gcc and install it as described here:

http://csjava.occ.cccd.edu/~gilberts/mingw/

After having added the mingw/bin directory to the PATH variable simply switch to your pjass development directory
in the command prompt and type mingw32-make to compile the executable.
You'll only want to edit misc.c, token.l, grammar.y, and pjass.res - all other files are generated.
If you're going to change token.l or grammar.y, you'll need flex/bison. Those are not included in MinGW.
You'll better get those from here: http://www.monmouth.com/~wstreett/lex-yacc/lex-yacc.html
Alternatively you can get Cygwin and add the cygwin/bin directory to your PATH variable.
Last but not least I've added version information to the executable, for compilation under Windows that is.
You'll need ResourceHacker which you can obtain here:

http://www.angusj.com/resourcehacker/

Extract it to a directory and update the RESHCK definition in the makefile to match that path.
If you're not compiling under Windows, remove the following line from the makefile:

$(RESHCK) -addskip $@.exe, $@.exe, resource.res ,,,

Current version is 1.0l, defined in pjass.res.

Additionally UPX is used to pack the executable. You can get it here:

http://upx.sourceforge.net

Good luck, PitzerMike