Write raw javac output to a log file

[mensinda]

Sometimes it is useful to look at the raw javac output when running in fork mode.

With this PR, the raw javac logs will be written to target/compiler-logs/javac_*.log

[olamy]

maybe it would be better as optional. when debug is activated?

[mensinda]

Making this optional would be fine, but I would not use any existing CompilerConfiguration, since my use case is easily inspecting the raw compiler output with the current configuration (well, CompilerConfiguration#isVerbose could work...). However, I also don't think that adding a new option to the CompilerConfiguration is a good idea, since (currently) only a specific usage of the javac compiler is supported.

Maybe this could be controlled with a system property?