If JAVA_HOME is incorrectly set to JRE instead of JDK, project will not compile

[blackears]

I was having some trouble compiling the getting started tutorial described on the JSweet homepage. After trying several things, I believe the problem may have been that my JAVA_HOME was incorrectly set to point to my JRE instead of the JDK. There was no obvious warning about this when I tried to compile the JSweet project. It would be helpful if a check for this could be put in the compile task and exit immediately with a warning if it is detected.

[kohlschuetter]

This should be fixed by using standalone compiler instead of relying on whatever JDK provides.
see PR #770 for details.