Before doing anything, I advise you to read the README and INSTALL-Sol2 files that come with the IP-Filter distribution.
Once you have fixed up the make file, you should be able to make the package as per the instructions in the INSTALL.Sol2 file. Make sure that you have /usr/ccs/bin in your PATH variable before wherever GNU-make is located. To make and install the package, you will need to su to root. When you su to root, you will probably need to add /usr/ccs/bin and your compiler and make directory to the root PATH variable, so that the strip command is available.
If you are a user of Solaris 7 and are using gcc 2.8.1 then you will encounter difficulties with the stdarg.h and vararg.h files. From the vantage point of Solaris 2.6, I can't provide a sensible solution. However, discussion on the IP-Filter mailing list mentions some work arounds: http://false.net/ipfilter/1999_02/0000.html1, http://false.net/ipfilter/1999_02/0096.html2, http://false.net/ipfilter/1999_02/0098.html3, http://false.net/ipfilter/1999_02/0099.html4, http://false.net/ipfilter/1999_02/0046.html5 and http://false.net/ipfilter/1999_02/0048.html6.