make JAVAC_OPTIONS configurable

The file Wrapping/Java/CMakeLists.txt contains

# javac runs out memory on Mac so increase heapsize
if(APPLE)
  set(JAVAC_OPTIONS -J-Xmx512m)
endif()

Now, that's a bit arbitrary. Why only on APPLE? Why 512m? In fact, on Debian we had to increase this to 1024m, see https://anonscm.debian.org/cgit/debian-science/packages/vtk6.git/tree/debian/patches/100_javac-heap.patch.

It'd suggest to make this configurable, perhaps via cmake -DJAVAC_OPTIONS=-J-Xmx512m.