A simple patch
Created attachment 90883 [details, diff] jss-3.4-jdk15.patch
There was an opened bug for this bug#119682 but it was not fixed.
Created attachment 92590 [details] jss-3.4-r1.ebuild generation-2 ebuild for jss
Your patch would have worked for me, but jss is broken when emerging on amd64. At least not when using sun-jdk (I now see that ibm has a 1.4 compatible amd64 compiler; I probably should have used that one...). The underlying cause is that jss links a native executable (64 bit on amd64), and if your jdk is 32bit you are out of luck - they can't be linked together. Anyway, I attached a ebuild that uses your patch and is gen2 compatible, so jss can be linked with a 64 bit java on amd64 machines. Finally, jss-3.4 is a bit dated: there is now jss-3.7. However, it seems that mozilla doesn't package the source any more - they expect folks to pull the source from CVS with a JSS_3_7_RTM tag. Retrofitting the ebuild to pull from CVS was a bit beyond my limited ebuild skills.
jss-3.4-r1 is migrated to generation 2, forces -target and -source to 1.4.